Ink jet recording apparatus

ABSTRACT

An ink jet recording apparatus which effects non-impact recording by the application of a jet of ink to a recording medium while the amount of ink to be jetted towards the recording medium is controlled to render it proportional to the recording velocity. For this purpose, the ink jet recording apparatus is provided with a feed-back loop system through which an electrical signal indicative of the pressure acting on the ink within an ink tank is fed back to a comparator to control operation of a compressed air source from which the pressure is applied to compensate for reduction in static pressure of the ink within the ink tank resulting from consumption of the ink. For maintaining the amount of the ink jetted proportional to the recording velocity, various arrangements are disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to an ink jet recording apparatus and,more particularly, to an ink jet recording apparatus having means forcontrolling the amount of ink jetted towards a recording medium.

An ink jet recording apparatus has hertofore been used as a device fordelineating a line, character or figure by the utilization of finedroplets of ink successively jetted from an ink tank through a finenozzle. By way of example, where this type of ink jet recordingapparatus is employed in an X-Y plotter, ink within the ink tank isjetted in the form of fine droplets through the nozzle at a relativelyhigh frequency towards the recording medium so that the ink dropletsdeposited on the recording medium will form a continuous line.

U.S. Pat. No. 3,968,498, patented on July, 1976, for example, disclosesan ink jet recording apparatus employed in an X-Y plotter. In general,since the ink jet recording apparatus intended for use in the X-Yplotter is required to satisfy the requirement that initiation andinterruption of the jetting of ink must precisely respond to thepresence and absence of a command signal (which corresponds to UP-DOWNsignals employed in an X-Y plotter of a type utilizing a writing pen,which UP-DOWN signals are selectively used to disengage and engage thetip of the writing pen from and to the recording medium), an ink jetrecording apparatus of the non-impact recording type wherein applicationof electric voltage between the nozzle and an electrode positioned inthe vicinity of the nozzle results in jetting of ink from the nozzletowards the recording medium is recommended for use in the X-Y plotter.

In the above numbered U.S. patent, the statement has been made that,where the ink jet recording apparatus is employed in the X-Y plotter, aresultant delineated line of uniform width cannot be obtained unless theamount of ink jetted is caused to vary with change in recordingvelocity. Starting from this problem, the above numbered U.S. patentdiscloses concrete means for solving the above described problem, suchas by the employment of a technique wherein the voltage to be appliedbetween the nozzle and the electrode is made proportional to therecording velocity, a technique wherein the frequency of vibrationsapplied to the ink within the ink tank is made proportional to therecording velocity or a combination of these techniques.

However, it has been found that the voltage applied between the nozzleand the electrode and the amount of ink jetted has a proportionalrelationship only within a limited range. Moreover, it has also beenfound that the employment of a method wherein vibrations or pulsatingpressures are applied to the ink within the ink tank has a disadvantagethat an exact proportional relationship cannot be obtained between thepressure and the amount of ink jetted and that the recording head andits related parts are adversely affected by such vibrations.

Furthermore, in the conventional method particularly wherein pressure isapplied to the ink within the ink tank during jetting of the ink, sinceactual measurement of the pressure thus applies is not made, a precisecontrol of the amount of ink jetted is not achieved. Moreover, theconventional ink jet recording apparatus wherein vibrations are employedto apply the pressure to the ink is not suited for delineating a linesince the frequency or speed of formation of the ink droplets is verylow.

Where control of the amount of ink jetted is effected in the manner asherein-above described in the conventional ink jet recording apparatus,no consideration has been paid to the relationship between the change inviscosity of the ink resulting from a change in temperature, the amountof ink within the ink tank, the pressure acting on the ink and thevoltage applied between the nozzle and the electrode, all of which havebeen found to affect the amount of ink being jetted from the nozzletowards the recording medium.

Moreover, in the case where the ink jet recording apparatus is to beemployed in the X-Y plotter, although the employment of a controltechnique necessary to control the amount of ink jetted so that a lineof uniform width can be obtained without being substantially adverselyaffected by the recording velocity and other factors provides thepossibility that lines of different width can be delineated on therecording medium one at a time with a single nozzle, such use has notyet been made of the conventional device.

In view of the foregoing, the use of a conventional ink jet recordingapparatus involves various problems which must be solved in order for itto be satisfactory for an X-Y plotter.

SUMMARY OF THE INVENTION

Accordingly, the present invention has for its essential object toprovide an ink jet recording apparatus of a type wherein means isprovided for varying the pressure acting on the ink to accuratelycontrol the amount of ink jetted.

Another object of the present invention is to provide an ink jetrecording apparatus of the type referred to above wherein the pressureacting on the ink can be varied in correspondence with the recordingvelocity to make the amount of ink jetted proportional to the recordingvelocity thereby controlling the amount of ink jetted so that a line ofuniform width can be delineated on the recording medium.

A further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above wherein means isprovided for preventing a change in viscosity of the ink incident to achange in temperature, or reduction in the amount of ink within the inktank, from adversely affecting the amount of ink jetted.

A still further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above wherein positive airpressure control means is provided for enabling the amount of ink jettedto be controlled by the pneumatic pressure acting on the top surface ofthe ink within the ink tank.

A still further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above which includes meansfor moving the ink jet recording apparatus from any point above therecording medium to a predetermined position, where the ink jetted doesnot adversely affect the recording medium, so that jetting of ink can beperformed on a trial basis.

A still further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above which can delineatelines of different width one at a time with only a single nozzle.

A still further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above in which the recordingmedium can be a highly electrically insulating film which is treated tohave an electroconductive property.

A still further object of the present invention is to provide an ink jetrecording apparatus of the type referred to above which basicallycomprises means for detecting the pressure acting on the surface of theink within the ink tank, means for generating an electric signal of avalue corresponding to a pressure which in turn corresponds to apreferred amount of ink to be jetted, means for comparing an outputsignal from the pressure detecting means with the signal from the signalgenerating means, and pressure control means responsive to the outputfrom the comparing means for controlling the pressure acting on the topsurface of the ink within the ink tank.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction withpreferred embodiments thereof with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic perspective view of an X-Y plotter employing anink jet recording apparatus of the present invention;

FIG. 2 is a top plan view of an ink jet generating unit employed in theink jet recording apparatus;

FIG. 3 is a side elevational view of the ink jet generating unit shownin FIG. 2;

FIG. 4 is a cross sectional view taken along the line IV--IV in FIG. 3;

FIG. 5 is a top plan view of an upper body of a recording head with anink tank removed;

FIG. 6 is a longitudinal sectional view of the ink tank;

FIG. 7 is a longitudinal sectional view of a lower body of the recordinghead;

FIG. 8 is a top plan view of the lower body of the recording head;

FIG. 9 is a longitudinal sectional view, on an enlarged scale, of aportion of the ink tank, showing the details of the structure in thevicinity of the tip of a nozzle carried by the ink tank;

FIG. 10 is a bottom plan view of a portion of the ink tank;

FIGS. 11 to 13 are enlarged partial sectional views illustrating thesequence in which one positioning pin secured to the ink tank is engagedin a groove in the lower body of the recording head to lock the ink tankto the recording head;

FIG. 14 is a graph showing the relationship between the amount of inkjetted from the nozzle and the pressure acting on the ink within the inktank;

FIG. 15 is a graph showing the relationship between the width of a linedelineated by the jet of ink issued from the nozzle and the amount ofthe ink jetted;

FIG. 16 is a schematic block diagram showing an X-Y servo-controlcircuitry employed in the X-Y plotter;

FIG. 17 is a schematic block diagram showing a circuitry for the ink jetrecording apparatus embodying the present invention;

FIG. 18 is a block diagram showing a velocity signal generating circuitforming a part of the circuit shown in FIG. 17;

FIG. 19 is a chart showing various waveforms of signals appearing in thecircuits of FIGS. 16 to 18;

FIG. 20 is a schematic diagram showing how a meniscus of ink at thenozzle tip is transformed into a jet of fine ink droplets in relation tothe voltage applied between the nozzle and the electrode;

FIG. 21 is a schematic top plan view of a compressed air source;

FIG. 22 is a side sectional view of the compressed air source shown inFIG. 22;

FIG. 23 is a end sectional view of the compressed air source taken online 23--23 of FIG. 21 showing a preferred manner in which a temperaturesensor is supported;

FIG. 24 is a graph showing the variation in pressure acting on the topsurface of the ink within the ink tank;

FIG. 25 is a schematic sectional view of a portion of the compressed airsource showing an example wherein a pressure regulator is employed foradjusting the pressure of the compressed air supplied to the ink tank;

FIG. 26 is a circuit diagram showing a gain control circuitry which maybe employed in the circuitry of FIG. 17;

FIG. 27 is a circuit diagram showing a modification of the gain controlcircuitry shown in FIG. 26;

FIGS. 28 and 29 are views respectively similar to FIGS. 21 and 22,showing a modified form of the compressed air source;

FIG. 30 is a block diagram showing a modified form of negative feed-backloop employed in the circuitry of FIG. 17;

FIG. 31 is a circuit diagram showing the details of the modifiednegative feed-back loop shown in FIG. 30;

FIG. 32 is a graph showing the relationship between the amount of theink jetted and the viscosity of the ink within the ink tank;

FIG. 33 is a schematic diagram showing the employment of a temperaturecompensating means which can be employed in the ink jet recordingapparatus embodying the present invention;

FIG. 34 is a diagram similar to FIG. 33, showing a modified form of thetemperature compensating means shown in FIG. 33;

FIGS. 35(A) and (B) are diagrams illustrating how variation in staticpressure of the ink within the ink tank affects formation of the jet ofink issued from the nozzle;

FIG. 36 is a circuit diagram showing a static pressure compensatingmeans which can be employed in the ink jet recording apparatus embodyingthe present invention;

FIG. 37 is a diagram similar to FIG. 36, showing a modified form of thestatic pressure compensating means;

FIG. 38 is a diagram similar to FIG. 36; showing a further modified formof the static pressure compensating means;

FIG. 39 is a circuit diagram showing a temperature compensating meanswhich can be employed in the ink jet recording apparatus embodying thepresent invention;

FIG. 40 is a circuit diagram showing a modified form of the temperaturecompensating means shown in FIG. 39;

FIGS. 41 to 44 are sectional views of the ink tank showing differentforms of a stabilizer accommodated within the tank for stabilizing thebody of ink within the ink tank;

FIGS. 45 to 47 are sectional views of a support for supporting therecording medium, showing various forms of construction thereofaccording to the present invention;

FIGS. 48 and 49 illustrate different manners of grounding the support;

FIG. 50 is a diagram similar to FIG. 16, showing a different X-Yservo-control circuitry employed in the X-Y plotter;

FIG. 51 is a diagram similar to FIG. 17, showing another embodiment ofthe circuiting ink jet for the recording apparatus of the presentinvention;

FIG. 52 is a return signal generating circuit forming part of thecircuit shown in FIG. 51;

FIG. 53 is a block diagram showing a test signal generating circuitwhich can be employed in the ink jet recording apparatus of the presentinvention;

FIG. 54 is a schematic perspective view of an X-Y plotter employing anink jet recording apparatus of a type having three nozzles which areselectively brought into operation for delineating lines of differentcharacteristics;

FIG. 55 is a schematic diagram of the ink supply system for the X-Yplotter of FIG. 55;

FIG. 56 is a schematic block diagram showing a circuitry employed in theX-Y plotter shown in FIGS. 54 and 55;

FIG. 57 is a block diagram showing a circuit for a nozzle cleaningdevice which can be employed in the ink jet recording apparatusembodying the present invention;

FIG. 58 is a block diagram showing a modification of the circuit for thenozzle cleaning device;

FIG. 59 is a diagram similar to FIG. 56, showing a further embodiment ofthe present invention wherein a different high voltage power source isemployed;

FIG. 60 is a block diagram showing the details of the power sourceemployed in the circuitry of FIG. 59;

FIG. 61 is a diagram similar to FIG. 17, showing a further embodiment ofthe present invention wherein a still further high voltage power sourceis employed;

FIG. 62 is a chart showing waveforms of signals appearing in the circuitof FIG. 61 which are shown in timed relation to each other;

FIG. 63 is a block diagram showing the details of a circuit for the highvoltage power source employed in the circuit of FIG. 61;

FIG. 64 is a diagram similar to FIG. 63, showing a modified form of thehigh voltage power source circuit of FIG. 63; and

FIGS. 65 and 66 illustrate photocouplers employed in input stages of aD.C. amplifier and switching circuit employed in the circuits of FIGS.63 and 64.

DETAILED DESCRIPTION OF THE INVENTION

Before the description of the present invention proceeds, it is to benoted that like parts are designated by like reference numeralsthroughout the accompanying drawings.

Referring now to FIG. 1, there is illustrated an X-Y plotterincorporating an ink jet recording apparatus according the presentinvention. In FIG. 1, the illustrated X-Y plotter has a pair of spacedparellel rails 3 and 4 rigidly mounted on a machine framework or bench2, one of said rails 3 having a toothed side and therefore constitutinga guide rack. Extending between the rack 3 and the rail 4 over a flatsupport 20 on the machine framework 2 is an X-axis carriage 5 supportedin position for movement in one direction parallel to the X-axis of thecoordinates and, particularly, parallel to the lengthwise direction ofthe rails 3 and 4. The X-axis carriage 5 has its opposed ends rigidlyconnected respectively to movable supports 6 and 7 which are movablymounted on the rack 3 and the rail 4. For effecting the movement of theX-axis carriage 5 in the X-axis direction, one of the movable supports 6which is mounted on the rack 3 has housed therein a servo-motor 8 havinga drive shaft (not shown) operatively engaged to the rack 3 through apinion (not shown) by way of any suitable transmission system fordriving the X-axis carriage 5 in the X-axis direction.

The X-axis carriage 5 carries a rack 9 rigidly mounted thereon andextending parallel to the lengthwise direction of the X-axis carriage 5.Mounted on the rack 9 for movement in a direction parallel to the Y-axisof the coordinates and, particularly, perpendicular to the direction ofmovement of the X-axis carriage 5, is a Y-axis carriage 10 havingtherein a servo-motor 11 having a drive shaft (not shown) engaged to therack 9 through a pinion (not shown) by way of any suitable transmissionsystem (not shown).

The Y-axis carriage 10 carries a recording head 12 rigidly securedthereto and supporting an ink jet generating unit 13 in a manner as willbe described in more detail later. Externally extending from a source ofcompressed air and a control circuitry, all housed within the machineframework 2, towards the ink jet generating unit 13 are a flexible tube18, through which compressed air is supplied to an ink tank 14, and abundle of high voltage supply cables 19 through which high voltage froma high voltage source is applied, the length of the tube 18 and thecable bundle 19 being so selected that the movement of the X-axis andY-axis carriages 5 and 10 will not be obstructed during a recordingoperation.

The flat support 20 on the machine framework 2 is so designed as tosupport thereon a recording medium 21, such as a sheet of paper, withoutpermitting the latter to be deformed and/or displaced during therecording operation.

The X-Y plotter thus far described is so designed that an intelligencesignal, generated from a source thereof, for example, a programmedcomputer, is delineated in the form of visual information such as lines,figures or characters, on the recording medium 21 while the angle ofrotation of any one of the servo-motors 8 and 10 is controlled toreflect the contents of the intelligence signal.

Referring to FIGS. 2 to 10, the ink jet generating unit 13 comprises theink tank 14 having a predetermined amount of ink 17 therein, a nozzle 15through which a series of fine droplets are jetted from the ink tank 14towards the recording medium 21, and a ring-shaped electrode 16positioned adjacent the nozzle 15. As best shown in FIGS. 4 and 6, theink tank 14 is in the form of a substantially cylindrical containerhaving one or more, for example, two, positioning pins 61 radiallyoutwardly protruding from the outer peripheral surface thereof, whichpins 61 are respectively engageable in mating grooves 71, formed in therecording head 12, in a manner as will be described later. As best shownin FIGS. 6 and 9, the ink tank 14 has a nozzle support 62, made ofmetallic material, having one end secured to the ink tank 14 and theother, free end thereof receiving the nozzle 15 which is also made ofmetallic material. Specifically, the nozzle 15 having one end solderedor welded to the nozzle support 62 in alignment with the hollow in saidnozzle support 62 extends downwards from said nozzle support 62 througha guide ring 63 of electrically insulating material secured or bonded tothe other end of the nozzle support 62. Surrounding of the nozzlesupport 62 is a protective sheath 64, made of electrically insulatingmaterial, such as synthetic resin, for example, formaldehyde polymer,one of the opposed ends of said protective sheath 64 adjacent the nozzle15 supporting the ring-shaped electrode 16. A seal ring 65 is providedbetween the bottom of the ink tank 14 and the adjacent end of the nozzlesupport 62 for avoiding any possible leakage of the ink 17 out of theink tank 14.

The recording head 12 is constituted by upper and lower bodies 12a and12b. The upper body 12a has a central bore 66 of a diameter slightlygreater than the outer diameter of the ink tank 14 and a pair of opposedguide grooves 67 extending completely through the thickness of the upperbody 12a in communication with the bore 66, which guide grooves 67respectively receive the positioning pins 61. As best shown in FIGS. 7and 8, the lower body 12b also has a central portion with upper andlower circular recesses 68 and 69 in alignment with each other, the bothbeing of a diameter slightly greater than the other diameter of the inktank 14. The upper circular recess 68 extends downwards from the uppersurface of the lower body 12b while the lower circular recess 69 extendsupwards from the lower surface of the same lower body 12b. Theserecesses 68 and 69 are connected to each other through an opening 70 ofa diameter slightly greater than the outer diameter of the protectivesheath 64, which opening 70 is defined in the lower body 12b. The lowerbody 12b has a pair of opposed guide grooves 71 defined therein incommunication with the recess 68 and in alignment with the guide grooves67 in the upper body 12a, and recess 68 has an enlarged area 68a of adiameter greater than the diameter of the remaining portion of saidrecess 68. A fixing ring 72 is accommodated within the enlarged area ofthe recess 68 for locking the ink jet generating unit 13 in position ina manner which will be described in more detail later.

The fixer ring 72 has a pair of pins 73 downwardly extending therefrom,one of these pins 73 being so connected with a spring member 74 that thefixing ring 72 is biased in one direction about the longitudinal axis ofthe opening 70 with the other of said pins 73 engaged with a releasebutton 75 as best shown in FIG. 8. This fixer ring 71 has a pair ofopposed cutout portions 76 for the passage of the positioning pins 61 onthe ink tank 14 therethrough when the ink tank 14 is mounted on therecording head 12, and inclined surfaces 77 downwardly inclined towardsthe respective cutout portions 76. Mounted on the bottom of the recess68 is a high voltage electrode ring 78 having an upper annular face towhich three electrode pieces 79 are soldered in equally spaced relationto each other and are held in position to contact a flange portion ofthe nozzle support 62, when the ink jet generating unit 13 is mounted onthe recording head 12, so that a high voltage can be applied to thenozzle 15 through a power supply line 81 having one end connected to aconnector 80 and the other end connected to the electrode ring 78, theintermediate portion of said line 81 being positioned in an appropriategroove defined in the head 12.

Positioned within the lower recess 69 is an electrode ring 82 having aninner peripheral face to which three electrode pieces 83 are solderedand from which said electrode pieces 83 extend downwards. Theseelectrode pieces 83 are made of metallic material having a sufficientelasticity so that these electrode pieces 83 can contact the ring-shapedelectrode 16 when the ink jet generating unit 13 is mounted on therecording head 12. For applying a voltage, opposed in polarity to thevoltage applied to the nozzle 15, to the ring-shaped electrode 16through the electrode ring 82 and then the electrode pieces 83, a leadline 84 is provided having one end connected to the connector 80 and theother end connected to the electrode ring 82, the intermediate portionof said line 84 being positioned in an appropriate groove defined in therecording head 12. It is to be noted that the electrode ring 82 and theelectrode pieces 83 are shielded by an outer protective sheath 60.

With the construction so far described, when the ink jet generating unit13 is inserted from above into the bore 66 with the positioning pins 61aligned with the guide grooves 67, the pins 61 contact the inclinedsurfaces 77 on the fixing ring 72 as shown in FIG. 11. As the jetgenerating unit 13 is further pushed, the fixing ring 72 is caused torotate in a direction against the bias of the spring member 74 with thepins 61 consequently caused to fall into the guide grooves 71 in thelower body 12b after having passed through the cutout portions 76 in thefixing ring 72 as shown in FIG. 12. Subsequent thereto, the fixing ring72 is rotated being biased by the spring member 74 whereby, as shown inFIG. 13, the pins 61 are confined within the guide grooves 71 in thelower body 12b of the recording head 12 so that the jet generating unit13 is held in position on the recording head 12 in the manner as shownin FIGS. 3 and 4.

The ink tank 14 includes a closure 86 detachably mounted on the topopening thereof, which closure 86 has a coupling 85 through which thetube 18 is connected to the interior of the ink tank 14.

Removal of the ink jet generating unit 13, including the ink tank 14,from the recording head 12 can be effected by applying an externalpushing force to the release button 75 to cause the fixer ring 72 torotate against the action of the spring member 74 due to the contact ofthe pin 73 by the inner end of the release button 75. By this action,the cutout portions 76 in the fixer ring 72 are brought immediatelyabove the positioning pins 61 in the ink tank 14, which are then engagedin the guide grooves 71, and, therefore, the jet generating unit 13 canreadily be removed from the recording head 12 by lifting it up.

The head 12 is also provided with a loupe 87 for facilitatingobservation of the ink being jetted from the nozzle 15 towards therecording medium 21. This loupe 87 comprises a reflecting mirror 87a andan optical system including an objective lens assembly and an eyepiecelens assembly and is detachably mounted on the recording head 12 by asupport bracket 89 having one or more fixtures 88, such as pin members.Specifically, the recording head 12 has holes 90 for receiving thefixtures 88 and a recess 91, the bottom of which recess 91 issubstantially rounded in complemental relation to the cylindrical bodyof the loupe 87. This loupe 87 can be mounted on the recording head bymoving the loupe 87 downwardly with a portion of the outer periphery ofthe loupe body in contact with the rounded bottom of the recess 91 untilthe fixtures 88 are substantially completely inserted into thecorresponding holes 91. It is to be noted that the ring-shaped electrode16 is, as best shown in FIGS. 9 and 10, provided with radially outwardlyextending jet observation slits 92 which are so designed as to provideno obstruction to the symmetrical development of an electrostatic field.

Referring now to FIG. 14, there is illustrated the graph showing arelationship between the amount of ink jetted from the nozzle 15 and thepressure acting on the top surface of the ink 17 within the tank 14. Inpreparing the graph of FIG. 14, the ink jet generating unit 13 havingthe construction as hereinbefore described has been used and thevariation in the amount of the jetted ink relative to variation in thepressure acting on the top surface of the ink within the ink tank 14 hasbeen plotted at intervals of one second while the voltage between thenozzle 15 and the ring-shaped electrode 16 was fixed. From the graph ofFIG. 14, it is clear that the amount of the jetted ink exhibits asubstantially proportional relation to the pressure acting on the inksurface and vice versa. Similarly, variation in the width of acontinuous line, drawn on a cylindrical recording medium being rotatedat a constant speed while the jet generating unit 13 was moved in adirection parallel to the axis of rotation of the cylindrical recordingmedium at a predetermined speed, was plotted relative to variation inthe amount of the jetted ink which was effected by supplying compressedair into the tank 14 to increase the pressure acting on the ink levelsurface, the result of which is shown in a graph of FIG. 15. From thegraph of FIG. 15, it is clear that the width of the delineatedcontinuous line exhibits a substantially logarithmically proportionalrelation to the amount of the jetted ink, and vice versa.

From the graphs of FIGS. 14 and 15, it is clear that the gradient of thestraight line interpolated in each of the graphs is determined by suchfactors as the physical properties of the ink used. Therefore, it can beunderstood that a precise control of the pressure acting on the inksurface within the ink tank 14 in relation to the recording velocitywill result in not only jetting of the ink in an amount appropriate tothe recording velocity, but also drawing of a continuous line of uniformwidth.

To this end, the ink jet recording apparatus embodying the presentinvention further comprises means for controlling the amount of inkjetted from the tank 14 through the nozzle 15 onto the recording medium21 in response to variation in the recording velocity, which controllingmeans is electrically associated with the X-Y control of the X-Y plotteras will now be described.

Referring to FIG. 16, there is illustrated an electric circuit blockdiagram of the X-Y servo-control of the X-Y plotter. As is well known tothose skilled in the art, the positioning of the recording head 12carrying the ink jet generating unit 13 used in the X-Y plotter requiresa precise control and, for this purpose, a negative feed-back system isemployed for the position control. It is, however, to be noted that theillustrated circuit block diagram is applicable only to the X-Y plotterof the type wherein electrical X-axis and Y-axis positioning signalsused to position the ink jet generating unit 13 relative to therecording medium 21 are employed in the form of pulses or digitalsignals and wherein the servo-motors 8 and 11 are each an analogservo-motor. It is also to be noted that, since the servo-control shownin FIG. 16 includes two separate circuit systems for the control of theX-axis servo-motor 8 and for the control of the Y-axis servo-motor 11,respectively, and these two circuit systems are substantially identicalin construction and operation, reference will be made only to the systemfor the control of the X-axis servo-motor 8 for the sake of brevity, thecorresponding parts of the system for the control of the Y-axisservo-motor 11 being identified by the prime (') affixed to likereference numerals.

The X-Y servo-control shown in FIG. 16 includes an interface 22 commonto both of the circuit systems referred to above, which interface 22generates, after the intelligence signal has been fed thereto from, forexample, a computer (not shown), X-axis and Y-axis positioning signalsSx and Sy in the form of digital signals which are respectively issuedfrom the interface 22 in association with the separate circuit systemsreferred to above. Considering the X-axis positioning signal Sx, thelatter is fed through a comparator 23 to a digital-analog converter 24wherein the digital signal from the interface 22 is converted into ananalog signal. This analog signal issuing from the converter 24 is,after having been amplified by an X-axis servo-amplifier 25, applied tothe X-axis servo-motor 8 to cause the latter to be rotated through anangle determined by the amplified analog signal. While the servo-motor 8is thus rotated, the velocity of rotation of the drive shaft of themotor 8 and the position of said drive shaft of the same motor 8 arerespectively detected by a velocity detector 26 and a position detector27. The velocity detector 26 may be composed of any known speedmeasuring instrument, for example, a tachometer coupled to the driveshaft of the motor and the position detector 27 may be composed of anyknown position detector, for example, a resolver. The output signal Vxfrom the velocity detector 26, which is indicative of the velocity ofrotation of the drive shaft of the motor 8, is in part applied to aninterface 31, described below, and in part fed-back to a stage precedingthe X-axis servo-amplifier 25 through a negative feed-back amplifier 28.On the other hand, the output signal from the position detector 27,which is indicative of the position of the drive shaft of the motor 8,is, after having been converted into a digital signal in ananalog-digital converter 29, fed back to the comparator 23 in which thedigital signal from the converter 29 is compared with the positioningsignal Sx for the purpose of position control of the X-axis carriage 5.

With respect to the system associated with the positioning signal Sy, itoperates in a substantially identical manner as hereinbefore describedand a similar output signal Vy from the velocity detector 26' is in partfed to the interface 31 and in part fed back to a stage preceding theY-axis servo-amplifier 25' while a similar output signal from theposition detector 27' is, after having converted into a digital signalin the analog-digital converter 29', fed back to the comparator 23'wherein said digital signal from the converter 29' is compared with thepositioning signal Sy for the purpose of position control of the Y-axiscarriage 10.

In addition to the servo-control of the construction as hereinbeforedescribed, the X-Y plotter embodying the present invention furtherincludes a pressure control for controlling the amount of ink jettedfrom the tank 14 towards the recording medium 21, the construction ofwhich will now be described in detail with particular reference to FIG.17 illustrating a block circuit diagram of the pressure control referredto above.

Referring now to FIG. 17, the interface 31 includes a velocity signalsynthesizer, a block diagram of which is best shown in FIG. 18, whichsynthesizes an output signal proportional to V=√Vx² +Vy², wherein Vrepresents the recording velocity of the recording head 12, morespecifically, the ink jet generating unit 13. The velocity signalsynthesizer comprises a pair of squaring circuits 32 and 33 connected inparallel to each other, which are in turn connected to an adder 34followed by a signal generator 35. Each of the squaring circuits 32 and33 is so designed that, when the output signal Vx or Vy from thevelocity detector 26 or 26' is applied thereto, it produces an outputsignal indicative of the second power of the velocity of rotation of thedrive shaft of the motor 8 or 11. The adder 34 performs an addition togive an output signal indicative of the sum of the second power of thevelocity of the drive shaft of the motor 8 and that of the motor 11,which is in turn applied to the signal generator 35 generating thesignal V which is equal to the square root of the sum of the secondpower of the velocity of the drive shaft of the motor 8 and that of themotor 11 as shown by the above formula. This corresponds to therecording velocity, of the ink jet recording head 12.

The velocity signal V thus synthesized in the interface 31 from thesignals Vx and Vy is fed to an analog comparator 36, which is composedof a differential amplifier the gain of which is one and which acts tocompare the velocity signal V with an analog output signal from apressure sensor 38. The pressure sensor 38 is used to detect thepressure of compressed air fed from an air pump 37 to the ink tank 14through the tube 18 and to generate an analog output signal indicativeof the pressure of the compressed air, which analog output signal is fedto the comparator 36 through a negative feed-back amplifier 39. Adifference signal emerging from the comparator 36, which is indicativeof the difference between the signal V and the analog output signal fromthe pressure sensor 38, is applied to a control circuit 40. The controlcircuit 40 may be composed of any known window comparator or a pulseconverter and is so designed as to generate two types of control signalsCW and CCW in synchronism with a clock pulse depending on whether thedifference signal from the comparator 36 exceeds a predetermined windowwidth set in the control circuit 40 or whether the difference signalfalls below the predetermined window width. More specifically, thecontrol circuit 40 generates the control signal CW when the differencesignal from the comparator 36 exceeds the value +V_(R) while itgenerates the control signal CCW when the difference signal falls belowthe value -V_(R). Although these control signals CW and CCW are appliedto a pulse motor drive circuit 41 one at a time, application of thecontrol signal CW to the drive circuit 41 results in a source 42 ofcompressed air being operated to increase the pressure of the compressedair fed to the ink tank 14 through the tube 18, and application of thecontrol signal CCW to the same drive circuit 41 results in thecompressed air source 42 being operated to decrease the pressure of thecompressed air fed to the ink tank 14 through the tube 18. However, itis to be understood that the control circuit 40 will not generate any ofthe control signals CW and CCW so long as the difference signal from thecomparator 36 remains within the predetermined window width between thevalues +V_(R) and -V_(R).

The pulse motor drive circuit 41 is so designed as to convert either ofthe control signals CW and CCw into a trigger signal utilizable to drivea pulse motor 43 and also to amplify the trigger signal. Specifically,the drive circuit 41 when receiving the control signal CW applies apositive trigger signal to the motor 43 to rotate the latter in onedirection, for example, a positive direction, and, when receiving thecontrol signal CCW, applies a negative trigger signal to the motor 43 torotate the latter in the opposite direction, that is, a negativedirection. As will be described in more detail later, rotation of thepulse motor 43 in the positive direction causes the air pump 37 toincrease the pressure of the compressed air while the rotation of thepulse motor in the negative direction causes the air pump 37 to decreasethe pressure of the compressed air.

The pressure sensor 38 is coupled to air pump 37 to detect the pressureof the compressed air issued from the air pump 37. As hereinbeforedescribed, the signal from the pressure sensor 38 which is indicative ofthe pressure of the compressed air issued from the air pump 37 is, afterhaving been amplified by the negative feed-back amplifier 39, fed backto the comparator 36 wherein it is compared with the velocity signal Vfor the purpose of pressure control. For example, if the value of thevelocity signal V becomes high, the value of the output signal from thepressure sensor 38 at this moment becomes lower than the value of thevelocity signal V and, therefore, the comparator 36 generates thedifference signal for supply to the control circuit 40 which in turngenerates the control signal CW to the drive circuit 41. The consequenceis that the compressed air source 42 is caused to operate to increasethe pressure of the compressed air fed to the ink tank 14.

It is to be noted that the intelligence signal applied from, forexample, the computer, to the interface 22 includes, in addition to anX-Y servo-control signal providing respective bases for the positioningsignals Sx and Sy, an ink jet trigger signal Sd. This ink jet triggersignal Sd is ultimately used for initiating and interrupting an ink jetfrom the nozzle 15 towards the recording medium 21 in synchronism withthe X-Y servo-control described above and is first applied to theinterface 31. It is further to be noted that this ink jet trigger signalSd functionally corresponds to a command signal, utilized in an X-Yplotter of the contact recording type wherein a recording pen isutilized, for engaging and disengaging the tip of the recording pen toand from the recording medium. As shown in FIG. 17, the ink jet triggersignal Sd is, after having been passed through the interface 31, appliedto a high voltage pulse generator 50, only when the necessity arises, tocause the latter to apply a high voltage switching signal to the nozzle15 of the ink jet generating unit 13, the consequence of which isgeneration of an ink jet directed from the nozzle 15 onto the recordingmedium 21.

FIG. 19 is a chart showing the relationship a with respect to time,among the signals Vx, Vy and V, the pressure P acting on the ink surfacewithin the tank 14, the voltage Vj applied to the nozzle 15, the widthof the resultant line delineated by the ink jet generating unit 13 andthe trigger signal Sd. In the graph of FIG. 19, during a period from themoment t₁ to the moment t₂, the recording head 12 is moved without theink jet being generated from the nozzle 15. In other words, during thisperiod t₁ to t₂, since the trigger signal Sd is not yet supplied eventhough the positioning signals Vx and Vy are being issued from theinterface 31, neither the pressure P nor the voltage Vj are respectivelyapplied to the ink surface within the tank 14 and the nozzle 15. Duringa period from the moment t₂ to the moment t₃, the recording head 12 ismoved in the direction parallel to the X-axis and, during a period fromthe moment t₄ to the moment t₅, the recording heat 12 is moved in thedirection parallel to the Y-axis. However, during the respective periodst₂ to t₃ and t₄ to t₅, the recording operation is performed by jettingof the ink. It is to be noted that the recording velocity during theperiod t₂ to t₃ is equal to that during the period t₄ and t₅.

During a period from the moment t₆ to the moment t₇, the recording head12 is moved simultaneously in the X-axis and Y-axis directions and,therefore, the recording velocity is higher than that during the periodt₂ to t₃ and also that during the period t₄ to t₅. Under this condition,the velocity signal V and the pressure P acting on the ink surfacewithin the ink tank 14 must accordingly be higher than that duringeither of the periods t₂ to t₃ and t₄ to t₅ and, by making the pressurehigher, the resultant line delineated on the recording medium 21 has auniform width.

From the chart of FIG. 19, it is clear that, when the intelligencesignal is entered into the interface 22, the X-Y servo-controlmechanism, the circuit diagram of which is shown in FIG. 16, is operatedto move the recording head 12 to a definite position relative to therecording medium 21. Upon subsequent entry of the trigger signal Sd, ajet of ink is produced from the nozzle 15 by the application of the highvoltage Vj to the nozzle 15 on one hand and the pressure of thecompressed air appropriate to the recording velocity V is applied to theink surface within the tank 14 on the other hand. Accordingly, theresultant line delineated by the ink jet from the nozzle 15 is uniformin width irrespective of the recording velocity V and continuation andinterruption of the ink jet issuing from the nozzle 15 satisfactorilyfollows the trigger signal Sd. Repeated cycles of issuance andinterruption of the ink jet issuing from the nozzle 15 can be rapidlyperformed by application of a switching voltage in accordance with theintelligence signal while a bias voltage is applied between the nozzleand an acceleration electrode.

FIG. 20 illustrates the sequence of formation of a jet of ink dropletswhich is produced by applying a bias voltage of 800 volts from thesource 30 of bias voltage to the ink jet generating unit to form ameniscus of ink at the tip of the nozzle 15 and by subsequently applyinga switching voltage of 1,000 volts from the high voltage pulse generator50, which switching voltage is superimposed on the bias voltage, whilethe pressure acting on the ink surface within the ink tank 14 ismaintained at a constant value equal to the atmospheric pressure. It hasbeen found that the meniscus at the tip of the nozzle 15 responded in 1to 3 milliseconds to the application of the switching voltage totransform the meniscus into a jet of droplets and the jet of dropletsdrawn from the nozzle 15 towards the recording medium 21 was interruptedin 0.5 millisecond subsequent to interuption of the supply of theswitching voltage. In view of the above, the recording apparatus of thepresent invention can be satisfactorily employed in an X-Y plotter dueto a quicker responsiveness than that of a conventional recordingapparatus of the contact recording type utilizing, for example, arecording pen.

The details of the compressed air source 42 referred to above inconnection with FIG. 17 will now be described with particular referenceto FIGS. 21 and 23. As hereinbefore described, the compressed air source42 includes the pulse motor 43 operable in response to the output fromthe drive circuit 41. In addition to the pulse motor 43, the compressedair source further includes a motion translator 46, composed of a rotarycam 44 rigidly mounted on the drive shaft of the pulse motor 43 and apiston rod 45 operatively coupled to said cam 44, for translating therotary motion of the drive shaft of the motor 43 into a linear motion ofthe piston rod 45. A piston 48 is rigidly mounted on one end of thepiston rod 45 remote from the rotary cam 44 and is reciprocally slidablyhoused within a cylinder 47. The air pump 37 forming a part of thecompressed air source 42 is constituted by a diaphragm member 49 havingan outer periphery secured to the cylinder 47 and a central portionsecured to the piston 48 and so disposed as to divide the interior ofthe cylinder 47 into first and second working chambers 47a and 47b, thefirst working chamber 47a accommodating therein the piston 48. Thepressure sensor 38 is disposed adjacent the cylinder 47 in position todetect variation in pressure within the second working chamber 47b inthe cylinder 47, which sensor 38 generates the electric signalindicative of the pressure of the compressed air to be fed, or beingfed, to the ink tank 14 through the flexible tube 18 having one endcoupled to the cylinder 47 in communication with the second workingchamber 47b and the other end coupled to the ink tank 14 in the manneras hereinbefore described.

The compressed air source 42 having the construction as hereinbeforedescribed is so designed as to operate in response to the negativefeed-back system wherein the electrical signal issuing from the pressuresensor 38 and indicative of the pressure of the compressed air fed tothe ink tank 14 is, after having been amplified by the negativefeed-back amplifier 39, compared with the velocity signal V in theanalog comparator 36.

With the compressed air source 42 so constructed as hereinbeforedescribed, it has been found that the compressed air pressure requiredin practice in order for the amount of ink jetted to substantiallycorrespond to the recording velocity as shown in the graph of FIG. 14shall be controlled to be in the range of 40 to 60 cmAq. Therefore, itcan be concluded that the maximum amount of change in volume of thecylinder 47 resulting from movement of the piston 48 within the cylinder47 need not be more than one-tenth of the total volume of the cylinder47. More particularly, assuming that the piston 48 is moving in acompression stroke, since the air within the cylinder 47 undergoes apolytropic change, the following equation can be established:

    P.sub.1 ·V.sub.1.sup.K =P.sub.2 ·V.sub.2.sup.K (1<K<1.4)

wherein P₁ and V₁ represent the pressure and volume of the air in thecylinder prior to being compressed, respectively, while P₂ and V₂represent the pressure and volume of the air in the cylinder subsequentto being compressed after the piston has moved a distance δl. Assumingthat V₁ -V₂ =δV wherein δV<<V₁, the value of P₂ can be expressed by thefollowing equation:

    P.sub.2 =P.sub.1 (1-δV/V.sub.1).sup.-K ≈P.sub.1 (1+K·δV/V.sub.1)

If the difference between the pressures P₂ and P₁ is expressed by δP,then, δP=P₂ -P₁ and, therefore, ##EQU1## If the effective surface areaof the diaphragm member 49 which receives the pressure is expressed byS, the following equation can be obtained:

    δV=S·δl

Accordingly, ##EQU2## Since (K·S·P₁)/V₁ is a constant, the quantity ofchange in pressure, that is, δP, is approximately proportional to thedistance δl of linear movement of the piston 48 during the compressionstroke. Moreover, since the pressure detected by the pressure sensor 38and the output voltage from the pressure sensor 38 exhibit a linearcharacteristic and since the cam 44 of the motion translator 46 is sodesigned as to make the relation between the angle of rotation of thepulse motor 43 and the stroke of movement of the piston a linearrelationship, the compressed air source 42 can be linearly controlled.

The compressed air source having the construction as hereinbeforedescribed is supported externally of the recording head 12 and isconnected with the ink jet generating unit 13 through the flexible tube18 extending between the cylinder 47 and the ink tank 14. Therefore, therecording head 12 for the support of the ink jet generating unit 13 canbe assembled or manufactured in a compact size without requiring anincreased weight. However, it is to be noted that, since the pressuresensor 38 tends to be unfavorably affected by vibrations which may occurduring the movement of the recording head 12, the connection between thepressure sensor 38 and the cylinder 47 should be by means of a flexibletube 51 and the sensor 38 should be mounted on a base through a dampingblock 52 made of rubber material so that substantially no vibration willbe transmitted to the pressure sensor 38, such as shown in FIG. 23.

Preferably, the flexible tube 18 extending between the cylinder 47 andthe ink tank 14 has the smallest possible inner diameter, for example, 1to 2 mm., so as to avoid any possible adverse influence on thecompressed air source system. More specifically, if the inner diameterof the flexible tube 18 employed is relatively large, intrinsicvibration of an air column determined by the distance between thediaphragm member within the cylinder and the ink surface within the inktank 14 (which distance may be 3 to 5 meters in a large-sized drawingmachine), such as shown by the curve b in a graph of FIG. 24, acts onthe ink surface within the ink tank 14, so that a change in pressure ofthe compressed air within the cylinder, such as shown by a curve a inthe graph of FIG. 24, is not accurately transmitted to the ink surfacewithin the ink tank 14. The consequence is that a close speed controlwill not be achieved. In view of the above, if the inner diameter of thetube 18 is made as small as possible, the undesirable influence of theintrinsic vibration can be minimized. It is to be noted that, if aneedle valve 53 is provided for manually adjusting the cross sectionalarea of the passage for the flow of the compressed air thereby adjustingthe resistance to the flow of the compressed air such as shown in FIG.25, the pressure acting on the ink surface within the ink tank 14 willbe as represented by the curve c in the graph of FIG. 24 which containsno vibration component. The employment of the needle valve 53 has beendeveloped to take advantage of the fact that flow of air within a pipeis often affected by the resistance due to the design of the pipe to theflow of the air and, therefore, by adequately adjusting the needle valve53, unwanted vibration can be substantially eliminated.

In the X-Y plotter utilizing the ink jet recording apparatus embodyingthe present invention, it is possible to cause the ink jet generatingunit 13 to delineate lines of different widths with only a singlenozzle. In order to achieve this, it is necessary to make the pressureof the compressed air, which is applied to the ink surface within theink tank 14, adjustable. This can be accomplished by adjusting theamplification gain of either the velocity signal V or the pressuresignal indicative of the compressed air pressure prior to applying thevelocity signal V or the pressure signal to the analog comparator 36.This will now be described with reference to FIG. 26 and FIG. 27.

Referring now to FIG. 26, there is illustrated a circuit which canadjust the amplification gain of the velocity signal V from theinterface 31 to the comparator 36. As shown, prior to the application ofthe velocity signal V to the comparator 36, the velocity signal V isapplied to a positive phase amplifier 55. The positive terminal of thepositive phase amplifier 55 is connected to the interface 31 through aresistor R, and the negative terminal of the amplifier 55 is groundedthrough a resistor R on one hand and connected to the comparator 36through a parallel circuit including a plurality of, for example, three,feed-back resistors Rx₁, Rx₂ and Rx₃ and associated selector switchesSW₁, SW₂ and SW₃ which are respectively connected in series with thefeed-back resistors Rx₁, Rx₂ and Rx₃.

Since the voltage gain of the positive phase amplifier 55 is determinedby (R+Rxn)/2R, the desired voltage gain can be achieved by closing anyone of the selector switches SW₁, SW₂ and SW₃ to complete a feed-backcircuit between the output of the amplifier to the negative terminal ofthe amplifier 55 through one of the resistors Rx₁, Rx₂ and Rx₃ which isconnected in series with the switch SW₁, SW₂ or SW₃ which is closed. Forselectively closing one of the selector switches SW₁, SW₂ and SW₃, aline width selection signal Swd is applied from an external circuit to again selection circuit 56 which is in turn connected to the selectorswitches SW₁, SW₂ and SW₃ so as to selectively close any one of theseselector switches depending on the contents of the line width selectionsignal Swd. Specifically, where a line of a relatively large width isdesired to be delineated on the recording medium 21, one of the selectorswitches SW₁ , SW₂ and SW₃, the closure of which causes an increase ofthe gain of the positive phase amplifier 55, is closed to complete thefeed-back loop through the associated resistor Rx₁, Rx₂ or Rx₃. In thiscase, the velocity signal V is, after having been amplified to representa higher recording velocity than the actual recording velocity, appliedto the comparator 36 through the positive phase amplifier, theconsequence of which is that the compressed air pressure is so increasedthat the line of relatively large width can be delineated by the ink jetgenerating unit 13 on the recording medium 21.

Where the recording velocity is variable, the arrangement may be suchthat the velocity signal indicative of the actual recording velocity isvaried by the line width selection signal Swd, which is in turnamplified by the gain selector circuit 56 prior to being applied to thecomparator 36. In other words, in the circuit of FIG. 26, the velocitysignal available when the recording velocity is constant has beendescribed as being applied to the comparator 36 while the gain of theamplifier 55 is varied. Accordingly, at the time a line of relativelylarge width, that is, a bold line, is desired to be delineated on therecording medium with respect to a certain velocity signal v_(a), theinput signal v_(b) applied to the comparator 36 has a relation expressedby v_(a) <v_(b). It is assumed that the actual recording velocity atthis time is Va. If the actual recording velocity is increased to avalue Vb which is higher than the value Va, the velocity signal nowrepresents the recording velocity v_(b). If this velocity signalindicative of the recording velocity Vb is decreased to the value v_(a)by the amplifier and, then, applied to the comparator 36, since thevalue Vb is higher than the value Va, the width of the resultantdelineated line can be increased in a similar manner as hereinbeforedescribed.

Summarizing the foregoing, the following table can be obtained.

    ______________________________________                                        Actual                 Velocity Signal                                        Recording     Velocity Emerging from                                          Velocity      Signal   Gain Selector                                                                             Line Width                                 ______________________________________                                        Method of                                                                             Va        v.sub.a       v.sub.a                                                                              Fixed                                  Fig. 16                                                                       Method of                                                                             Va        v.sub.a  v.sub.b                                                                            v.sub.b > v.sub.a                                                                    Bold                                   Fig. 26                         v.sub.b < v.sub.a                                                                    Fine                                   Modified                        Vb < Va                                                                              Bold                                   Method of                                                                             Vb        v.sub.b  v.sub.a                                                                            Vb > Va                                                                              Bold                                   Fig. 26                         Vb > Va                                                                              Fine                                   ______________________________________                                    

Referring to FIG. 27, there is illustrated a circuit for adjusting theamplification gain of the pressure signal supplied from the pressuresensor 38 to the comparator 36. The circuit shown in FIG. 27 isconstituted by components similar to those employed in the circuit ofFIG. 26 and is operable in a substantially similar manner to the circuitof FIG. 26 except for a difference in performance is that, whereas inthe circuit of FIG. 26 the voltage gain is increased when a bold line isdesired to be delineated, in the circuit of FIG. 27 the voltage gain ofthe amplifier 57 is descreased where a bold line is desired to bedelineated.

In either of the circuits of FIGS. 26 and 27, it is to be noted that,although circuit has been described as employing the separate,parallel-connected resistors Rx₁, Rx₂ and Rx₃ for selectively varyingthe amplification gain of the amplifier 55 or 57, a single variableresistor may be employed in place of the separate resistor, in whichcase the width of the line to be delineated can be varied.

Instead of employing an electrical circuit, such as shown in FIG. 26 orFIG. 27, for enabling the ink jet generating unit 13 to delineate lineof different width according to a line width selection signal Swd, amechanical means may be employed which will now be described withparticular reference to FIGS. 28 and 29.

Referring now to FIGS. 28 and 29, there is shown an auxiliary pump 59positioned adjacent and in parallel relation to the cylinder 47. Thisauxiliary pump 59 is constituted by a cylinder having a piston 94reciprocally slidably accommodated within the interior of the cylinder.Extending externally of the cylinder 95 from the piston 94 is a rack 96constantly engaged with a drive pinion 97 mounted on the drive shaft ofa pulse motor 58, which pulse motor 58 can be energized by the lineselection signal. The interior of the cylinder, the volume of whichvaries depending upon the position of the piston 94, is communicated tothe working chamber 47b within the cylinder 47 by way of a connectionpipe 93.

In the construction thus far described, it will readily be seen thatstepwise rotation of the pulse motor 58 in response to the line widthselection signal applied thereto results in correspondingly stepwisemovement of the piston 94, the consequence of which is that thecompressed air within the cylinder 95 is fed stepwise to the workingchamber 47b. It is to be noted that the pulse motor 58 is energized onlywhere a line of different width is desired to be delineated on therecording medium through the ink jet generating unit 13. On the otherhand, the compressed air pump 37 is utilized only for the purpose ofcontrolling the amount of the jetted ink in such a manner as to avoidany fluctuation thereof with respect to the recording velocity.

With the pressure control system shown in FIG. 17 wherein the negativefeed-back control system is employed, the pressure sensor 38 employed isrequired to be of a type having a high sensitivity because the pressureacting on the ink, that is, the pressure to be controlled is usually ata low level, for example, 0 to 100 cmAq. Therefore, a semiconductorstrain gauge may be employed for the pressure sensor. However, apressure sensor utilizing a semiconductor strain gauge tends to have aninferior temperature characteristic and the output voltage availablefrom such a pressure sensor is relatively low, for example, some tenmillivolts. In view of this, where a pressure sensor utilizing asemiconductor strain gauge is employed, the negative feed-back amplifiermust be of a type having a relatively high amplification factor. Thus,the use of a pressure sensor utilizing a semiconductor strain gauge notonly causes a problem associated with the inferior temperaturecharacteristic of the strain gauge, but also a problem associated withthe requirement of high skill and expensive parts needed in themanufacture of the negative feed-back loop system.

On the other hand, the time during which the X-Y plotter is operated perdrawing is usually short, for example, several minutes to some tenminutes in the case where it is used in making a drawing, and X-Yplotter will be satisfactory if it can be operated only for that time.Accordingly, so far as the above described time during which the X-Yplotter can be satisfactorily be operated is involved, any change intemperature will be no more than several degrees C. and, therefore, ifthe negative feed-back loop is designed as shown in either of FIGS. 30and 31, any disturbance resulting from a change in temperature canadvantageously be avoided.

Referring first to FIG. 30, the circuit shown in FIG. 17 is shown tohave an input gate 98 inserted in a line which connects the interface 31to the comparator 36 for supplying the velocity signal V. It is to benoted that the negative feed-back amplifier is, in the feed-back loopshown in FIG. 30, replaced by a differential amplifier 39'. Theamplifier 39' has an output terminal connected to the comparator 36through a feed-back gate 100, a positive input terminal connected to thepressure sensor 38 and a negative input terminal connected to an offsetadjusting circuit 99. The output terminal of the differential amplifier39' is also connected to a zero value detector 101 which is in turnmechanically coupled to the input gate 98 and also the feed-back gate100, the output signal from the detector 101 being utilized as aswitching control signal for the gates 98 and 100. It is to be notedthat the amplifier 39' has a design wherein the level of the outputgenerated by the amplifier 39' so long as the system is in a standstillposition is zero. In order for an operator to be able to see whether ornot the output level of the differential amplifier 39' is zero, the zerovalue detector 101 may includes a pilot lamp for the display of theoutput level of the amplifier 39'.

The operation of the system shown in FIG. 30 will now be described.Assuming that the compressed air source 42 is not operated and nocompressed air is fed to the ink tank 14, if the output level of theamplifier 39' is not zero due to a drift occurring in either or both ofthe pressure sensor 38 and the amplifier 39' resulting from atemperature change, the zero value detector 101 generates a switchingcontrol signal by which the gates 98 and 100 are operated to complete acircuit between the ground and the positive terminal of the comparator36 and also a circuit between the ground and the negative terminal ofthe comparator 36, respectively, as shown in FIG. 30, so that the supplyof the velocity signal V to the comparator 36 is interrupted on one handand the feed-back loop is isolated. At the same time, the pilot lamp maybe lit by the output from the detector 101 to show that the output levelof the amplifier 39' is not zero. When the operator, responding to thelighted pilot lamp, subsequently manipulates the offset adjustingcircuit 99 so that the level of the input to the negative terminal ofthe amplfier 39' is adjusted sufficiently to cause the output level ofthe amplifier 39' to become zero, the detector 101 upon detection of thezero level output generates another switching control signal by whichthe gates 98 and 100 are again operated to complete a circuit betweenthe interface 31 and the comparator 36 and also a circuit between thecomparator 36 and the amplfier 39', respectively, on one hand and thepilot lamp is also extinguished to inform the operator that the outputlevel of the amplifier 39' has become zero, in which condition thepressure control system employing the feed-back loop as shown in FIG. 30operates in a manner similar to that shown in FIG. 17.

In practice, each of the gates 98 and 100 is preferably constituted byan analog switch operable by a digital signal (TTL level) which theoffset adjusting circuit 99 is preferably constituted by a voltagefollower having a positive input terminal connected to a manuallyadjustable potentiometer 106 as shown in FIG. 31. The zero valuedetector 101 is preferably constituted by a window comparator having apredetermined window width within the range of ±v_(a) volt which issufficiently low as compared with the required control precision.

The operation of the pressure control system of FIG. 17 in which thefeed-back loop of the circuit shown in FIG. 31 is employed will now bedescribed. Prior to the X-Y plotter being operated, the cylinder workingchamber 47b (FIG. 22) is communicated to the atmosphere through anormally opened, electromagnetically operated valve (not shown), whichelectromagnetically operated valve is subsequently closed when the X-Yplotter starts its operation, to confine the working chamber 47b withinthe cylinder 47. It is to be noted that, so long as the working chamber47b is communicated to the atmosphere, the level of the input to thepressure sensor 38 is zero. If the output level of the differentialamplifier 39' is within the range of ±v_(a) volt subsequent to theclosure of the electromagnetically operated valve, the windowcomparator, that is, the detector 101, causes the analog switches, thatis, the gates 98 and 100, to complete the circuit between the comparator36 and the differential amplifier 39' and also the circuit between theinterface 31 and the comparator 36, thereby causing the pressure controlsystem to operate.

However, in the event that the output level of the amplifier 39' isoutside the range of ±v_(a) volt, due to a temperature drift occurringin the pressure sensor 38 and the amplifier 39', although the inputlevel of the pressure sensor 38 is still zero, the detector 101 causesthe gates 98 and 100 to interrupt the supply of the velocity signal fromthe interface 31 to the comparator 36 and also to disconnect thefeed-back loop as shown so that the pressure control system ceases itsoperation. This condition can be indicated to the operator by the pilotlamp. When the operator in response to the ignited pilot lampmanipulates the potentiometer 106 of the offset adjusting circuit 99 toset the output level of the amplifier 39' to a value equal to or lessthan v_(a) volt, the detector 101 then causes the gates 98 and 100 tocomplete the circuit between the interface 31 and the comparator 36 andthe circuit between the comparator 36 and the amplifier 39', therebycausing the pressure control system to operate.

In this manner, precise pressure control can be achieved during theoperation of the X-Y plotter. Accordingly, during the time during whichthe X-Y plotter is operated, for example, some ten minutes, the pressuresensor can be satisfactorily be employed even if it is of a type havingan inferior temperature dependence characteristic, substantially withoutrequiring any expensive parts and without causing any substantialreduction in the overall performance of the pressure control system.

The ink jet generating unit 13 embodying the present invention may havea temperature compensation means for completely avoiding any possiblevariation in width of the line being drawn on the recording medium whichmay otherwise result from a change in the physical properties of the inkused under the influence of the ambient temperature. More specifically,it is well understood that the viscosity of the ink 17 within the inktank 14 tends to be lowered with increase in the temperature of the inkused. As the viscosity of the ink used is lowered because of theincreased temperature of the ink, the amount of the ink jetted from thenozzle 15 towards the recording medium 21 increases in inverseproportion to the decrease in viscosity of the ink as evidenced by thecurve in the graph of FIG. 32. In view of this, even though thedimensions of the nozzle 15, the type of ink employed, the voltageapplied between the nozzle 15 and the ring-shaped electrode 16 and thepressure applied on the ink surface within the ink tank 14 remain thesame, it is apparent that, if the ambient temperature is reduced to suchan extent that the viscosity of the ink employed is appreciablyincreased, the amount of ink jetted from the nozzle 15 onto therecording medium 21 will be correspondingly reduced and, if the ambienttemperature is increased to such an extent that the viscosity of the inkemployed is appreciably decreased, the amount of ink jetted from thenozzle 15 onto the recording medium 21 will be correspondinglyincreased. This means that the width of the line being delineated ordrawn on the recording medium and the density of ink deposits formingsuch delineated line vary in proportional relation to the ambienttemperature. The employment of the temperature compensation meansreferred to above can substantially eliminate the foregoing disadvantageand may be designed such as shown in FIG. 33 or FIG. 34.

Referring first to FIG. 33, the temperature compensating means is shownto comprise a transducer 109 including a temperature sensor 108 mountedon the ink tank 14 in position to detect the temperature of the ink 17within the tank 14, said transducer 109 generating an electric signalindicative of the temperature of the ink 17 detected by the temperaturesensor 108. The bias voltage source 30' employed in the example of FIG.33 is of a type having a voltage regulator which is electrically coupledto the transducer 109 so that output voltage from the bias voltagesource 30' can be adjusted in response to variation in temperature ofthe ink 17 within the ink tank 14. By way of example, the arrangement issuch that, if the temperature of the ink 17 within the tank 14 islowered, the voltage regulator in the bias voltage source 30' isoperated to increase the output voltage from said source 30' and, if thetemperature of the ink 17 becomes high, the voltage regulator in thebias voltage source 30' is operated to decrease the output voltage fromsaid bias voltage source 30'. Therefore, it will readily be seen that,so long as the trigger signal applied to the high voltage pulsegenerator 50 is constant, the high voltage pulse generator 50 generates,in response to the output voltage from the bias voltage source 30', ahigh voltage pulse corresponding to the temperature of the ink 17 sothat, irrespective of the temperature of the ink 17 within the ink tank14, a substantially constant amount of ink will be jetted from thenozzle 15 towards the recording medium 21.

Referring now to FIG. 34, the output signal from the transducer 109,which is indicative of the temperature of the ink 17 within the tank 14,is applied to the control circuit 40 so that the bias level of thetrigger signal emerging from the control circuit 40 is varied accordingto the temperature of the ink 17 within the ink tank 14 in such a mannerthat, when the temperature of the ink 17 is low, the compressed airsource 42 is operated to increase the pressure of the compressed airbeing applied to the ink surface within the ink tank and, when thetemperature of the ink 17 is high, the compressed air source 42 isoperated to decrease the pressure of the compressed air being applied tothe ink surface within the ink tank 14. Even with the circuit shown inFIG. 34, it is clear that, as long as the output signal from thecomparator 36 being applied to the control circuit 40 remains constant,a substantially constant amount of ink will be jetted from the nozzle 15towards the recording medium 21 irrespective of variation in temperatureof the ink 17 within the inkk tank 14.

It is to be noted that the circuit shown in FIG. 33 may be combined withthe circuit shown in FIG. 34. For example, in the circuit of FIG. 34,while the bias voltage source is employed in the form of a bias voltagesource of a type having a voltage regulator such as shown in FIG. 33,the transducer 109 may be electrically connected not only to the controlcircuit 40, but also to the voltage regulator in the bias voltagesource.

Furthermore, the ink jet recording apparatus of the present inventionmay have means for compensating for a reduction in the static pressureof the ink within the ink tank 14 resulting from consumption of the ink.As is well known to those skilled in the art, as the ink within the inktank is consumed during the recording operation performed by the X-Yplotter, not only is the amount of ink jetted no longer proportional tothe recording velocity, but also formation of ink droplets at the nozzletip at the start of jetting thereof towards the recording medium isadversely affected.

The first mentioned problem may be negligible because the pressureacting on the ink surface is sufficiently higher than the staticpressure due to the amount of ink within the ink tank. However, withrespect to the second mentioned problem, because the pressure acting onthe ink surace at the start of jetting of the ink from the nozzle tip isalways zero, that is, equal to the atmospheric pressure acting on thenozzle tip, the static pressure of the ink within the ink tank tends toadversely affect the formation of the ink droplets to be jettedsuccessively from the nozzle tip. More specifically, assuming that theink tank contains a sufficient amount of ink, as shown in FIG. 35(A), asufficient meniscus is first formed at the nozzle tip, as shown by (a),prior to the application of the switching voltage to the nozzle, whichmeniscus is subsequently transformed, as shown by (b), into a pluralityof ink droplets directed towards the recording medium upon applicationof the switching voltage, a constant jetting of ink being, as shown by(c), established one to three milli-seconds subsequent to theapplication of the switching voltage.

Contrary to the manner of formation of the ink jet as shown in FIG.35(A), if the ink tank contains an insufficient amount of ink, themanner of formation of the ink jet tends to follow the sequence shown inFIG. 35(B). Referring now to FIG. 35(B), as shown by (a), aninsufficient meniscus is first formed at the nozzle tip prior to theapplication of the switching voltage and, even though a sufficientmeniscus is subsequently formed as shown by (b), it cannot be readilyexpelled from the nozzle tip upon application of the switching voltageor, if expelled, it tends to result in a jet of ink travelling along apath displaced from its normal course of travel.

In view of the above, the employment of the static pressure compensatingmeans according to the present invention is advantageous in that theabove described disadvantages can substantially be eliminated.

Where the static pressure compensating means is employed, it isnecessary to detect any variation in the static pressure of the inkwithin the ink tank 14. This detection can be accomplished by detectingthe angle of rotation of the drive shaft of the pulse motor 43 employedin the compressed air source 42. This is because the fact that thereduction of the amount of ink within the ink tank 14 correspondinglyresults in a reduction in static pressure as the air space above the inksurface within the ink tank increases even though the level of thevelocity signal remains the same, and the angle of rotation of the driveshaft of the pulse motor 43 is increased in response to this reductionin static pressure of the ink within the ink tank 14 to cause thecompressed air to be incrementally supplied to the ink tank so that thepressure proportional to the velocity signal can be applied to the inksurface of the ink within the ink tank 14. In other words, the ink jetrecording apparatus of the present invention is designed such that theangle of rotation of the drive shaft of the pulse motor 43 graduallyincreases in response to consumption of the ink within the ink tank 14so that a pressure proportional to the velocity signal is applied to theink level surface so that the amount of ink appropriate and proportionalto the recording velocity is jetted towards the recording medium.Therefore, it is clear that the angle of rotation of the drive shaft ofthe pulse motor 43 substantially corresponds to the reduction of theamount of the ink within the ink tank 14, that is, the reduction instatic pressure on the ink within the ink tank 14.

The static pressure compensating means shown in FIG. 36 comprises apotentiometer 110 operatively coupled to the pulse motor 43 fordetecting the angle of rotation of the drive shaft of the pulse motor43, a peak value hold circuit 111 for detecting and holding the peakvalue of the potentiometer 110, and inverting amplifier 112 forinverting and amplifying the output signal from the hold circuit 111,and a level shift circuit 113 operable by the inverted output from theinverting amplifier 112. The output signal from the level shift circuit113 is applied to the analog comparator 36. The level shift circuit 113is composed of a positive phase amplifier the voltage gain of which isone and the level of an output voltage from the circuit 113 is shiftedby the output from the inverting amplifier 112. Detection of the angleof rotation of the drive shaft of the pulse motor 43 may be made interms of a value relative to a certain reference signal indicative ofthe recording velocity and, therefore, in the circuit shown in FIG. 36,the angle of rotation of the drive shaft of the pulse motor is detectedin terms of a value relative to the maximum recording velocityattainable by the ink jet recording unit 13. This is because, in the X-Yplotter, the maximum recording velocity is usually fixed.

The operation of the circuit shown in FIG. 36 will now be described.

Assuming that the static pressure of the ink within the ink tank 14remains at a predetermined value, the level shift circuit 113 permitsthe passage of the velocity signal from the interface 31 to thecomparator 36 so that the pulse motor 43 is rotated. The angle ofrotation of the drive shaft of the pulse motor 43 being rotated isdetected by the potentiometer 110, the output from said potentiometer110 indicative of the angle of rotation of the drive shaft of the pulsemotor 43 being detected and held in the peak value hold circuit 111. Theoutput signal from the hold circuit 111 is, after having been invertedand amplified by the inverting amplifier 112, applied to the negativeterminal of the level shift circuit 113. It is to be noted that theoutput voltage from the potentiometer 110 during this condition is suchthat the output from the inverting amplifier 112 is zero, that is, aground potential.

If the static pressure of the ink 17 within the ink tank 14 issubsequently reduced with the consequent increase of the angle ofrotation of the drive shaft of the pulse motor 43, the output voltagefrom the potentiometer 110 becomes high and, consequently, the outputfrom the peak value hold circuit 111 correspondingly increases. Theresult is that the output level of the inverting amplifier 112 becomesnegative, which is in turn applied to the level shift circuit 113. Thelevel shift circuit 113 serves to shift the negative output level fromthe inverting amplifier 112 to a positive output level, the consequenceof which is that the pulse motor 43 can be rotated further until theoutput voltage from the potentiometer 110 becomes high. Accordingly, thecircuit including the potentiometer 110, the hold circuit 111, theinverting amplifier 112 and the level shift circuit 113 constitutes apositive feed-back loop and the position where the drive shaft of thepulse motor 43 is stopped can be determined in relation to the negativefeed-back loop including the pressure sensor 38 and the feed-backamplifier 39. The gain of each of the positive and negative feed-backloops can be determined empirically. Furthermore, since the rate ofreduction of the amount of the ink within the ink tank 14 is very low,the frequency characteristic, that is, the interrupting frequency, ofthe positive feed-back loop is preferably selected to be 1 Hz or less.

In the circuit shown in FIG. 36, the peak value hold circuit 111 mayinclude a warning device for issuing a warning signal Sa indicative ofthe fact that substantially all the ink within the ink tank 14 has beenconsumed and the ink surface is positioned flush with or below theentrance leading into the nozzle, the generation of such warning signalSa being by detecting the maximum possible angle of rotation of thedrive shaft of the pulse motor 43.

The circuit shown in FIG. 36 is such that a D.C. bias component of thevelocity signal indicative of the recording velocity is modulated by theangle of rotation of the drive shaft of the pulse motor 43, which isindicative of the reduction in amount of the ink within the ink tank 14,to effect the static pressure compensation.

Where the feed-back amplifier 39 employed in the circuit of FIG. 17 isreplaced by a differential amplifier 39' having the positive inputterminal connected to the pressure sensor 38, a series circuit includingthe potentiometer 110 and the peak value hold circuit 111 may beconnected between the pulse motor 43 and the negative input terminal ofthe differential amplifier 39' through a positive phase amplifier 114,as shown in FIG. 37. In the circuit shown in FIG. 37, if the staticpressure of the ink 17 within the ink tank 14 remains at thepredetermined value, the level of the output from the positive phaseamplifier 114 is zero, that is, a ground potential and, therefore, theoutput signal from the pressure sensor 38 is applied to the analogcomparator 36 through the differential amplifier 39'. However, as thestatic pressure of the ink within the ink tank 14 decreases duringconsumption of the ink 17, the output from the peak value hold circuit111 is increased and the level of the output from the positive phaseamplifier 114 becomes positive. Since the positive output from thepositive phase amplifier 114 is an inverted input signal to thedifferential amplifier 39', the level of the output from the pressuresensor 38 is, after having passed through the differential amplifier39', lowered and, therefore, in a similar manner as with the circuitshown in FIG. 36, the pulse motor 43 is rotated in such a direction thatthe output from the potentiometer 110 is increased.

Furthermore, instead of the angle of rotation of the drive shaft of thepulse motor 43 being detected as a parameter indicative of reduction instatic pressure of the ink within the ink tank 14, the time during whichthe recording operaton is performed may provide a similar parameter.Where the recording time or period is taken as a parameter indicative ofreduction in static pressure of the ink within the ink tank 14, thecircuit arrangement shown in FIG. 38 may advantageously be employed.

Referring now to FIG. 38, the static pressure compensating means showncomprises a counter 117, the output of which is zero at the time the inktank 14 contains a sufficient and predetermined amount of ink therein.The counter 117 is electrically connected to a digital-analog converter118 which is in turn connected to the negative input terminal of thelevel shift circuit 113 through the inverting amplifier 112. As long asthe output from the counter 117 remains zero, the output from either theconverter 118 or the inverting amplifier 112 is equally zero, that is, aground potential and, therefore, the velocity signal from the interface31 is, without being level-shifted by the level shift circuit 113,applied to the analog comparator 36.

However, the pulse generator 50 upon receipt of the trigger signal Sdthrough the interface 31 applies the switching voltage to the nozzle 15to initiate jetting of ink within the ink tank 14. When this continuesfor a substantial period of time, the ink level within the ink tank 14is lowered due to the consumption of the ink 17 and there is anaccompanying reduction in static pressure of the ink 17 within the inktank 14. Since this reduction in static pressure of the ink within theink tank 14 is, in the case where the amount of ink jetted remains thesame, proportional to the time during which the trigger signal Sd isapplied to the high voltage pulse generator 50, an AND gate 116 havingseparate input terminals to which the trigger signal Sd from theinterface 31 and clock pulses from a clock pulse generator 115 arerespectively applied is triggered on to allow the passage of the clockpulses therethrough to the counter 117 only during the duration of thetrigger signal Sd, the clock pulses being, after having passed throughthe AND gate 116, counted by the counter 117. Therefore, it is clearthat the output signal from the counter 117 is indicative of therecording time. The output signal indicative of the recording time is,after having been converted by the converter 118 and then inverted andamplified by the inverting amplifier 112, applied to the level shiftcircuit 113. The signal indicative of the recording time is, whenapplied to the level shift circuit 113, shifted to a positive level sothat the pulse motor 43 can be rotated in such a direction that theamount of compressed air to be fed to the ink tank 14 can be increased.

In the circuit shown in FIG. 27, the warning signal Sd referred to abovein connection with the circuit of FIG. 37 can be issued from the counter117. Although any of the clock pulse generator 115, counter 117 andconverter 118 may be of a type commercially available, a clock pulsegenerator capable of generating clock pulses the frequency of which is0.1 Hz and a counter and converter having 12 bits are preferred.

With the static pressure compensating means shown in any one of FIGS. 36to 38, although the recording time varies depending upon the volume ofthe ink tank 14 and/or the amount of ink jetted per unit time, it hasbeen found that the recording time extends over about 3 hours accordingto an experiment wherein ink having a specific gravity of 0.9, chargedinto an ink tank having a volume of 5 ml., was jetted at a rate of 0.4mg/sec. onto a recording medium while the recording velocity was 20m/min. The 3 hours' recording time resulted in delineation of a lineextending about 4 km.

In addition to the static pressure compensating means described above,the ink jet recording apparatus of the present invention may have meansfor compensating for variation in the amount of compressed air suppliedinto the ink tank 14, which variation may result from an increase intemperature of the compressed air being produced in the cylinder 47. Asis well known to those skilled in the art, any machine when operated fora substantial period of time generates a heat energy and the ink jetrecording apparatus of the present invention is no exception.Particularly, the cylinder 47 and the pulse motor 43 tend to generateheat during the continued operation of the apparatus, which heatenergies is then transmitted to the compressed air being produced withinthe cylinder 47. With the temperature compensating means shown in FIGS.39 and 40, even though the compressed air supplied to the ink tank 14expands under the influence of the elevated temperature, undesiredvariation in pressure of the compressed air can advantageously becompensated for. This will now be described with reference to FIGS. 39and 40.

Referring first to FIG. 39, the temperature compensating means shown inFIG. 39 is illustrated as combined with the static pressure compensatingmeans shown in FIG. 36 and comprises a series circuit composed of atemperature sensor 119, positioned to detect the temperature within thecylinder of the air pump 37, such as a thermistor, a transducer 120 forconverting the output from the temperature sensor 119 into an electricsignal indicative of the temperature within the cylinder of the air pump37 and an inverting amplifier 121 for inverting and amplifying theelectrical signal from the transducer 120. This series circuit isconnected to the inverting amplifier 112 and the output terminal of theamplifier 121 is connected to the positive input terminal of theamplifier 112.

The operation of the circuit shown in FIG. 39 will now be described.Assuming that the static pressure of the ink within the ink tank 14remains at a predetermined value, the level shift circuit 113 permitsthe passage of the velocity signal from the interface 31 to thecomparator 36 so that the pulse motor 43 can be rotated. The angle ofrotation of the drive shaft of the pulse motor 43 being rotated isdetected by the potentiometer 110, the output from said potentiometer110 indicative of the angle of rotation of the drive shaft of the pulsemotor 43 being detected and held in the peak value hold circuit 111. Theoutput signal from the hold circuit 111 is, after having been invertedand amplified by the inverting amplifier 112, applied to the negativeterminal of the level shift circuit 113. It is to be noted that theoutput voltage from the potentiometer 110 during this condition is suchthat the output from the inverting amplifier 112 is zero, that is, aground potential.

If the static pressure of the ink 17 within the ink tank 14 issubsequently reduced with the consequent increase of the angle ofrotation of the drive shaft of the pulse motor 43, the output voltagefrom the potentiometer 110 becomes high and, consequently, the outputfrom the peak value hold circuit 111 correspondingly increases. Theresult is that the output level of the inverting amplifier 112 becomesnegative, which is in turn applied to the level shift circuit 113. Thelevel shift circuit 113 serves to shift the negative output level fromthe inverting amplifier 112 to a positive output level, the consequenceof which is that the pulse motor 43 can be rotated further until theoutput voltage from the potentiometer 110 becomes high. Accordingly, thecircuit including the potentiometer 110, the hold circuit 111, theinverting amplifier 112 and the level shift circuit 113 constitutes apositive feed-back loop and the position where the drive shaft of thepulse motor 43 is stopped can be determined in relation to the negativefeed-back loop including the pressure sensor 38 and the feed-backamplifier 39. The gain of each of the positive and negative feed-backloops can be determined empirically. Furthermore, since the rate ofreduction of the amount of the ink within the ink tank 14 is very low,the frequency characteristic, that is, the the interrupting frequency,of the positive feed-back loop is preferably selected to be 1 Hz orless.

Assuming now that the series circuit including the temperature sensor119, the transducer and the inverting amplifier 121 is not provided, anincrease in the temperature of the compressed air produced by the airpump 37 results in an increased pressure acting on the ink surfacewithin the ink tank 14 from the pressure normally corresponding to theangle of rotation of the drive shaft of the pulse motor 43, theconsequence of which is that the angle of rotation of the drive shaft ofthe pulse motor 43 is reduced by the operation of the negative feed-backloop for the detection of that pressure and there is no output from theinverting amplifier 112.

The series circuit shown in FIG. 39 serves to compensate therefor. Inother words, in the event that the temperature of the compressed airbeing produced by the air pump 37 increases, the angle of rotation ofthe drive shaft of the pulse motor 43 is reduced by the operation of thenegative feed-back loop, resulting in an increased level of the outputfrom the inverting amplifier 112, as hereinbefore described. On theother hand, the series circuit including the inverting amplifier 121then operates in such a manner that the inverting amplifier 112generates an output, the level of which is decreased in proportion tothe temperature detected by the temperature sensor 119 to an extentsufficient to compensate for the reduction in the output level of theinverting amplifier 112 which has occurred in the manner as hereinbeforedescribed. Therefore, the output signal supplied from the invertingamplifier 112 to the level shift circuit 113 acts on the velocity signalsupplied to the level shift circuit 113 so that the ink within the inktank 14 will not be adversely affected by the compressed air having theincreased temperature and the pulse motor 43 is rotated through an anglesufficient to compensate for the reduction in amount of the ink withinthe ink tank 14.

It is to be noted that the warning signal Sa as described in connectionwith the circuit of FIG. 36 is, in the circuit shown in FIG. 39, issuedfrom the inverting amplifier 112.

Where the temperature compensating means is desired to be combined withthe static pressure compensating means shown in FIG. 37, what isnecessary is to connect the inverting amplifier 121 to the negativeterminal of the positive phase amplifier 114 so that the output from thepeak value hold circuit 111 can be temperature-compensated as shown inFIG. 40. The circuit shown in FIG. 40 functions in a substantiallysimilar manner to the circuit shown in FIG. 39.

The ink tank 14 employed in the ink jet recording apparatus of theinvention may have a stabilizer for substantially avoiding variation instatic pressure of the ink resulting from wavy motion of the ink surfacewithin the ink tank. As is well known to those skilled in the art, theink jet recording apparatus when employed in the X-Y plotter is moved inall directions and, therefore, the ink within the ink tank 14 undergoesmotion. This motion of the ink tends to cause variation in staticpressure particularly when the ink surface within the ink tank 14 isdisturbed, which in turn results in the recording apparatus delineatinga line of varying width even through the overall amount of ink jettedremains the same throughout the recording operation.

The stabilizer used in the ink tank 14 according to the presentinvention substantially eliminates the influence of the motion of theink within the ink tank 14 on the static pressure thereof.

Referring first to FIG. 41, the stabilizer is shown to be constituted bya wad 122 of metallic or plastic fibers accommodated within and,therefore, immersed in the ink within the ink tank 14. In the exampleshown in FIG. 42, the stabilizer is shown to be constituted by aplurality of metallic meshes 123 accommodated within the tank 14 andstacked one above the other.

FIG. 43 illustrates an example wherein the metallic meshes 123 shown inFIG. 42 are arranged within the ink tank 14 in equally spaced relationto each other.

It is to be noted that the stabilizer shown in any one of FIGS. 41 to 43serves not only as means for stabilizing the body of ink within the inktank 14 during movement of the ink jet generating unit 13, but also as afilter for preventing any solid foreign matter contained in the ink fromentering the nozzle 15.

The stabilizer may be, as shown in FIG. 44, constituted by a float 124having a shape similar to the cross sectional area of the ink chamber ofthe tank 14. Preferably, the float 124 is made of foamed polyethylene,cork or any other suitable material having a specific gravity less thanthat of the ink used.

By the employment of the stabilizer of the construction shown in any oneof FIGS. 41 to 44, it has been found that the ink within the tank 14 andretained in position by the stablizer can be caused to withstand anacceleration of about 2G acting on the ink incident to movement of theink jet generating unit 13 with no substantial wavy motion occurringtherein. The fact that substantially no motion of the body of the inkwithin the ink tank 14 occur when the stabilizer is present has theeffect that the jetting of ink from the nozzle 15 towards the recordingmedium is advantageously stablized so that a line of uniform width canbe delineated on the recording medium.

Referring back to FIG. 1, either or both of the recording medium 21 andthe support 20 for the support of the recording medium 21 thereon is,when employed in the X-Y plotter incorporating the recording apparatusof the present invention, preferably of a type having anelectroconductive property. The reason for this will now be described.

In the case where the recording medium which is a synthetic resin sheet,for example, polyester film, having a relatively low electroconductivityon the order of 10₁₄ Ω surface resistance is used in combination with anink which has an electrostatically chargeable property, since the inkjetted from the nozzle 15 constitutes a series of ink droplets eachhaving an electrostatic charge thereon and travelling towards therecording medium, the fine ink droplets first deposited on the polyesterfilm tend to interfere with or repel the fine ink droplets subsequentlydeposited on the same polyester film. The consequence is that one ormore ink smears tend to be formed on the delineated line and/or thatsome of the ink droplets being jetted towards the polyester film tend tobe electrostatically attracted back towards the grounded ring-shapedelectrode 16, thereby soiling the electrode 16. This is because, sincethe polyester film or other recording medium having a lowelectroconductivity is used for the recording medium 21 in connectionwith the ink jet recording apparatus provided by the present invention,the potential charge on the fine ink droplets is not discharged orgrounded.

Moreover, even though the recording medium 21 used has at least anappreciable electroconductivity, a similar phenomenon is liable to occurunless the flat support 20 for the support of the recording medium 21 iselectroconductive. Specifically, in this case, the potential charge onthe ink droplets is, when the ink droplets are deposited on theelectroconductive recording medium, conducted to the electroconductiverecording medium and remains without being discharged or grounded. Theconsequence is that the potential on the surface of the recording mediumis increased to such an extend that the directionality of the inkdroplets subsequently jetted may be adversely affected.

On the other hand, depending upon the purpose for which the recordingmedium on which information, such as a drawing, is desired to bedelineated, the demand for the use of a sheet or film made of syntheticresin, such as polyester, polyethylene, polyamide; polyvinyl fluoride ora plastic material prepared from cellulose, has increased. Since thesheet or film of the type referred to above has a very lowelectroconductivity, for example, 10¹⁴ Ω in terms of surface resistance,a fine line of uniform width can be delineated thereon only withdifficulty, even though every component of the ink jet recordingapparatus functions satisfactorily, for the reason which has beendescribed hereinabove.

In order to make possible the use of such low electroconductiverecording medium in connection with the ink jet recording apparatus, oneor both of the methods, which respectively pertain to improvement in therecording medium and the support 20 for the support of the recordingmedium thereon, may be employed, which will now be described.

The method associated with the recording medium is the employment of arecording medium of a type;

(1) having one or both surfaces coated with a hydrophilic material, suchas silicon oxide, a surface active agent or a powder ofelectroconductive material, or

(2) having a surface active agent or a powder of electroconductivematerial admixed therein during the manufacture of the recording medium,or

(3) having one or both surfaces vapour-plated with a metallic materialsuch as Al, Cu, In₂ O₃, Fe, Co, Ni, Ru, Rh, Rd, Os, Ir or Pt.

Examples of the surface active agents referred to above are anionicsurfactants such as sulfonates, sulfates, and the other acid esterderivatives; nonionic surfactants such as an ether type surfactant,including ethoxylated alkylphenols, ethoxylated aliphatic alcohols andpolyoxyalkylene oxide block copolymers, ester type surfactant, includingpolyoxyethylene fatty acid esters and carboxylic esters, amide typesurfactants, including polyoxyethlene fatty acid amides, ether and estertype surfactants, including ethoxylated anhydrosorbitol esters and thelike; cation surfactants such as alkyl guaternary ammonium salts, alkylimidazolines, polyalkylene polyamine derivatives, and the other aminederivatives amphoteric surfactants such as betaines and polymersurfactant such as acuylic acid derivatives.

Examples of the powder of electroductive material referred to above arepowdered argentum, powdered cuprum and carbon black.

Although the application of a surface active agent referred to abovedoes not appear to impart an electroconductive property to the recordingmedium, the presence of the surface active agent in the recording mediumcauses the latter to be wetted, or to be able to absorb water from theair, to such an extent that the recording medium or at least one surfaceof the recording medium can be considered electroconductive. It shouldbe noted that the terms "electroconductivity" and "electroconductiveproperty" hereinabove and hereinafter used in connection with therecording medium thus treated are to be understood as meaning areduction in resistance relative to the intrinsic resistance of therecording medium. By way of example, it has been found that theapplication of a hydrophilic material to one surface of a polyethylenesheet having a surface resistance of 10¹⁴ Ω resulted in a reduction inthe resistance to 10⁷ to 10¹⁰ Ω and, therefore, the polyethylene sheetafter having the hydrophilic material applied thereto can be consideredelectroconductive relative to its condition before the application ofsuch hydrophilic material.

The other method associated with the support 20 is to make the support20 substantially electroconductive. For this purpose, the support 20 maybe made of any metallic material. However, where the use of metallicmaterial as a material for the support 20 is undesirable, any ofconstructions shown in FIGS. 45 to 47 may be employed, which will now bedescribed.

Referring first to FIG. 45, the support 20 is shown to comprise a pairof rubber plies 125 stacked one above the other with a mesh of anelectroconductive wire 126 sandwiched firmly therebetween. Theelectroconductive wire 126 sandwiched may be made of iron, copper ornicrome wire and is arranged in a mesh-like configuration. Where thenicrome wire is employed for the electroconductive wire 126, anadditional advantage will be obtained if electric current is suppliedthrough the electroconductive wire 126. This is because the flow ofcurrent through the wire 126 results in emission of heat from the wire126 which is in turn transmitted through the upper rubber ply 125 to therecording medium so that ink deposits on the recording medium canreadily be dried.

Referring to FIG. 46, the support 20 is shown to comprise a singlerubber layer 127 having admixed therein a powdered electroconductivematerial, for example, carbon or silver, in an amount such that theelasticity of the rubber layer will not appreciably be destroyed.Preferably, the amount of the powdered electroconductive material isselected so as to give the support 20 a specific resistance not morethan 10⁵ Ω.cm.

Referring to FIG. 47, the support 20 is shown to comprises an upperrubber layer 125' having an undersurface to which another rubber layer127' having admixed therein a powdered electroconductive material, forexample, carbon or silver, is bonded. The support 20 having constructionshown in FIG. 47 may be considered a combined version of one of therubber plies 125 of FIG. 45 and the rubber layer of FIG. 46. Where thesupport 20 of the construction shown in FIG. 47 is employed in the X-Yplotter, the support should be placed on the machine framework with therubber layer 125' facing towards the ink jet generating unit 13.

The construction shown in either of FIGS. 45 and 47 is preferred wherethe powdered electroconductive material when admixed in the rubber layertends to impart a colour other than white to the support whereas thesupport 20 is desired to be white.

Irrespective of the construction of the support shown in FIGS. 45 to 47,where the potential evolved in the ink deposits is desired to bedischarged only through the electroconductivity of the recording mediumtreated as hereinbefore described, it is necessary to connect therecording medium, supported on the support 20, to the ground by wiringsuch as shown by the chain line in FIG. 48. This can readily be achievedby placing one end of the grounded wire on the surface of the recordingmedium with or without the aid of a brush electrode or a weight held incontact with said surface of the recording medium.

Where the recording medium treated as hereinbefore described is combinedwith the support 20 having the construction shown in any one of FIGS. 45to 47, one or both of the recording medium 21 and the support 20 may beelectrically connected to the ground as shown in FIGS. 48 and 49.Referring to FIG. 48, there is shown a wire 126 in the form of a wiremesh sandwiched between the rubber plies 125 connected to the ground asshown by the solid line. Similarly, where the support 20 has theconstruction shown in either of FIGS. 46 and 47, the electroconductiverubber layer 127 or 127' should be connected to the ground. However, itshould be noted that, where the support 20 is connected to the ground, arecording medium having a high insulating value, that is, which is notone of the above described types (1) to (3), may satisfactorily andeffectively be employed. In other words, even though the recordingmedium which has not been treated to impart to it any electroconductiveproperty is placed on the support 20, which is made of any metallicmaterial or of the construction shown in any one of FIGS. 45 to 47 andwhich is, therefore, connected to the ground, discharge of the potentialof the ink deposits to the ground can satisfactorily and effectively beachieved. This is substantially shown in FIG. 49.

In FIG. 49, while the insulating recording medium 21 is shown to besupported on the support 20 having the construction shown in FIG. 45,the wire 126 in the form of a wire mesh is shown to have one endconnected to the ground and the other end connected to the groundthrough a heating power source P.W.

While in the example of FIG. 48 the ground potential should be equal tothe potential supplied to the ringshaped electrode 16, the groundpotential in the example of FIG. 49 should be lower than the potentialsupplied to the ring-shaped electrode 16. The heating power source P.W.is preferably of a type capable of supplying a voltage not more than 50volts so that the potential supplied from the power source P.W. to thewire 126 will not adversely affect the jet of ink being issued from thenozzle 15.

It should be noted that the arrangement shown in FIG. 49 is equallyusuable with a support 20 having construction shown in FIG. 46 or FIG.47.

While the support 20 may be made of any metallic material ashereinbefore described, the use of rubber to form part or all of thesupport as shown in FIGS. 45 to 47 is preferred. This is because thesupport having construction shown in any one of FIGS. 45 to 47 can beused not only in an X-Y plotter utilizing the ink jet recordingapparatus, but also in the conventional X-Y plotter utilizing arecording pen.

The X-Y plotter employing the ink jet recording apparatus according tothe present invention may include a nozzle cleaning device having aconstruction which will be hereinafter described. As is well known tothose skilled in the art, the ink used in the ink jet recordingapparatus has a certain viscosity and, despite this fact, the ink of thetype referred to above is required to have a quickdrying capability.Therefore, where the ink jet generating unit is not operated for asubstantial period of time, there is the possibility that a portion ofthe ink within the ink tank 14 which remains within the nozzle 15 willdry up and clog the ink passage within the nozzle 15. If this actuallyhappens, subsequent jetting of the ink from the ink tank 14 through thenozzle 15 will be adversely affected.

The above is true even where the recording apparatus includes aplurality of ink jet generating units each having a construction ashereinbefore fully described, for carrying out a multi-color recordingoperation wherein the ink jet generating units are brought intooperation one at a time or for the purpose of delineating, one at atime, lines of different width by the selective use of the ink jetgenerating units.

The provision of the nozzle cleaning device referred to above isadvantageous in that the above described disadvantages can besubstantially eliminated. Cleaning of the clogged nozzle can be achievedby forcing ink within the nozzle to be jetted or discharged on a trialbasis prior to an actual recording operation. In order that the trialjetting of the ink within the nozzle will not soil the recording medium,it is necessary to return or move the recording head to a predeterminedposition where the trial jetting of the ink is permitted. For thispurpose, the nozzle cleaning device which will subsequently be describedincludes a return signal generating unit having a construction shown inFIG. 52.

It is, however, to be noted that the nozzle cleaning device is includedin the interface 31 of the circuitry for the ink jet recording apparatusof FIG. 51, which is similar to the circuit of FIG. 17.

Referring now to FIG. 52, the return signal generating unit comprises adifferential circuit 200 adapted to receive an initiating signal Sowhich is generated from the interface 22 of the X-Y servo controlcircuitry of FIG. 50, which is the same as that of FIG. 16 with addedmeans in the interface at the time the recording head 12 starts to movein readiness for an actual recording operation with the ink being jettedfrom the nozzle 15. The differential circuit 200 generates an outputsignal in response to the start of the initiating signal so appliedthereto from the interface 22. The output signal from the differentialcircuit 200 is fed to an OR circuit 201.

The initiating signal is also applied to an AND gate 202 which passesclock pulses from a clock pulse generator 203 only during the durationof the initiating signal So. Assuming that the ink jet generating unit13 is held at a standstill for a period of time longer than apredetermined time, in which condition no trigger signal Sd is generatedfrom the interface 22 of the servo control circuitry of FIG. 50 not,therefore, supplied to a NOT circuit 204, an AND gate 205 to which theclock pulses emerging from the AND gate 202 are supplied is triggered onto pass such clock pulses to a counter 206. The counter 206, uponreceipt of the clock pulses from the AND gate 205, starts counting thenumber of the clock pulses supplied thereto in synchronism with aswitch-off of the trigger signal Sd. It is to be noted that the counter206 can be reset to clear the contents stored therein by a reset signalwhich is generated from a differential circuit 207, for differentiatingthe start of the output from the NOT circuit 204, each time the triggersignal is switched off.

An output signal from the counter 206, which is indicative of the numberof the clock pulses counted, is compared in a comparison circuit 208with a reference value set in the comparison circuit 208. The comparisoncircuit 208, when the number of the clock pulses counted exceeds thepredetermined number represented by the reference value set in thecomparison circuit 208, generates an output signal which is supplied toa differential circuit 209. The output from the comparison circuit 208,after having been differentiated by the differential circuit 209 andafter having subsequently been passed through the OR circuit 201,becomes the return signal Sr which is in turn applied to the interface22 of the servo control circuitry of FIG. 50.

Upon receipt of the return signal Sr from the interface 31 of FIG. 51,the X-Y servo-control mechanism causes the recording head 12 to returnor move to the predetermined position where no recording medium ispresent above the support 20 and generates an end signal Se which issupplied to the interface 31, which end signal Se is indicative ofcompletion of the return movement of the recording head 12 to thepredetermined position.

The nozzle cleaning device further includes a test signal generatorelectrically connected to the return signal generating unit, theconstruction of which test signal generating unit will now be describedwith reference to FIG. 53.

Referring to FIG. 53, the test signal generating unit, which is ininterface 31, comprises a test signal generator 210 which, upon receiptof the end signal Se supplied from the X-Y servo control circuitry shownin FIG. 52, applies an output signal to a pressure control signalgenerator 211, a second gate drive circuit 212, a differential circuit213 and an OR gate 214, all being electrically connected in parallelwith to each other. The second gate drive circuit 212 is operativelyassociated with a second gate circuit 215 having a pair of fixedcontacts 215a and 215b, contact 215a being electrically connected to afirst gate circuit 216 which will be described later, and the contact215b being electrically connected to an output terminal of the signalgenerator 211, and a movable contact 215c coupled to the second gatedrive circuit 212 in such a manner that, during a period in which theoutput signal, that is, the test signal, is applied to the second gatedrive circuit 212 from the test signal generator 210, the movablecontact 215c is engaged with the fixed contact 215b to pass a pressurecontrol signal from the generator 211 to the comparison circuit 36 (FIG.51). This pressure control signal from the generator 211 serves asimilar purpose as achieved by the velocity signal V and, therefore, thepressure can ultimately be applied to the ink surface within the inktank 14. However, the pressure to be applied to the ink surface withinthe tank 14 and produced in response to the pressure control signal fromthe generator 211 is preferably higher than the pressure to be appliedto the ink surface within the tank 14 and produced in response to thevelocity signal V as hereinbefore described.

On the other hand, the test signal from the generator 210, when appliedto the OR gate 214, passes through the gate 214 as a start signal Sswhich is applied to the high voltage pulse generator 50 to effect thejetting of ink from the nozzle 15 with the recording head 11 held at thepredetermined position. The differential circuit 213, after havingdetected the end test signal supplied thereto from the test signalgenerator 210, generates an output signal Sj, indicative of the end oftrial jetting of ink, which is supplied to the interface 22 of the servocontrol circuitry of FIG. 50.

The first gate circuit 216 has a pair of fixed contacts, contact 216abeing electrically connected to receive the velocity signal and thecontact 216b being electrically connected to the ground, and a movablecontact 216c electrically connected to the second gate circuit 215, saidmovable contact 216c being operatively coupled to a first gate drivecircuit 217 in such a manner that, during a period in which the triggersignal Sd is applied to the drive circuit 217, the movable contact 216cis engaged with the fixed contact 216a to pass the velocity signal Vthrough the first gate circuit 216. During the actual recordingoperation, the velocity signal V is successively passed through thefirst and second gate circuits 216 and 215 to the comparison circuit 36on one hand and the trigger signal Sd is passed through the OR gate 214to the high voltage pulse generator 50 to operate the ink jet generatingunit 13, as hereinbefore described.

Shown in FIGS. 54 to 57 is a nozzle cleaning device utilizable in an X-Yplotter employing the ink jet recording apparatus of a type utilizing aplurality of ink jet generating units each having a construction ashereinbefore described. More particularly, as best shown in FIGS. 54 and55, the recording heat 12' mounted on the Y-axis carriage 10 is shown tohave a plurality of, for example, three, ink jet generating units 13a,13b and 13c, each comprising an ink tank 14a, 14b or 14c, a nozzle 15a,15b or 15c and a ring-shaped electrode 16a, 16b or 16c. These ink jetgenerating units 13a to 13c can have the same construction as that ofthe ink jet generating unit 13 mounted on the recording head 12 ashereinbefore described, but are shown to have the nozzles 15a to 15c ofdifferent inner diameter, the nozzle 15a having a larger inner diameterthan that of the nozzle 15b which in turn has an inner diameter largerthan that of the nozzle 15c. This is because the recording apparatusshown in FIGS. 55 to 57 is intended for use in delineating lines ofdifferent widths one at a time. The employment of the nozzles 15a to 15cof different inner diameter is based on the fact that the amount of inkjetted is proportional to the fourth power of the inner diameter of thenozzle divided by the length of such nozzle.

For selectively bringing any one of the ink jet generating units 13a to13c into operation, a selection signal Ses fed from the interface 31 isutilized. As best shown in FIGS. 55 and 56, this selection signal Ses isapplied to a unit selecting circuit 218, connected in the circuitbetween the high voltage pulse generator 50 and a group of the nozzles15a to 15c, and also to a valve selecting circuit 219 for selecting theoperation any one of electromagnetically operated valves EV₁, EV₂ andEV₃ connected between the compressed air source 42 and the group of theink jet generating units 13a to 13c. This selection signal Ses issynthesized from a linewidth selection signal Sdd inputted to theinterface 31 from the X-Y servo-control circuitry shown in FIG. 16. Thelinewidth selection signal Sdd may be in the form of binary-codeddecimal digits where the number of the ink jet generating units is threesuch as shown.

Referring particularly to FIG. 57, upon generation of the linewidthselection signal Sdd from the X-Y servo-control circuitry shown in FIG.16, this linewidth selection signal Sdd is supplied through an OR gate220 to the differential circuit 200 and then to the OR gate 201 fromwhich the return signal Sr is issued. As hereinbefore described withreference to FIG. 52, the return signal Sr is used to return therecording head 12' to the predetermined position for trial jetting ofink from the nozzle of one of the ink jet generating units 13a to 13cwhich is assigned to operate by the linewidth selection signal Sdd.

Since the linewidth selection signal is always generated from the startof operation, the trial jetting of ink with the recording head held atthe predetermined position can be performed with the circuit of FIG. 57.Where the jetting of ink is interrupted for a period of time longer thanthe predetermined time, the trial jetting of ink can be performed in amanner similar to that described with reference to FIGS. 52 and 53.

A circuitry shown in FIG. 58 is a combination of the circuitries ofFIGS. 52 and 53 and has been developed on the basis of the finding that,since any possible adverse effect of drying-up of the ink within thenozzle which may occur where the jetting of ink is interrupted for aperiod of time longer than the predetermined time is slight and sincethe number of operations is small, the trial jetting of ink can beperformed merely by applying a voltage without the pressure of thecompressed air being relied on. The circuitry of FIG. 58 is so designedthat, upon generation of the linewidth selection signal, the compressedair and the voltage are simultaneously applied to effect jetting of ink,and where the jetting of ink is interrupted for a period of time longerthan the predetermined time, only the voltage is applied to effect atrial jetting of ink while the recording head is held at thepredetermined position. To this end, only when the output signal isissued from the differential circuit 200 which has detected a variationin the linewidth selection signal Sdd, is the pressure control signalfrom the pressure control signal generator 211 generated. Therefore,when the linewidth selection signal Sdd varies, the return signal Sr isissued from the OR gate, as hereinbefore described with reference toFIG. 53, and, at the same time, the output from the differential circuit200 is applied to the pressure control signal generator 211. In responseto the output from the differential circuit 200 thus applied to thepressure control signal generator 211, the pressure control signalgenerator 211 generates a pressure control signal and supplies it to thesecond gate circuit 215 and, when the test signal is issued from thetest signal generator 210 indicating that the recording head has beenreturned to the predetermined position, the pre-sure control signalpasses through the second gate 215. The pressure control signal passingthrough the second gate circuit 215 is used to control the operation ofthe compressed air source 42 to apply the pressure to the ink tank ashereinbefore described. At the same time, the voltage is also applied.Since the return signal which is generated when the period of timeduring which the jetting of ink is interrupted exceeds the predeterminedtime does not accompany the output from the differential circuit 200,only the voltage is applied in this case.

It is to be noted that the circuitry shown in FIG. 58 is equally beapplicable to the embodiment shown in FIGS. 50 to 53.

While in the circuitry shown in any one of FIGS. 50 to 53, FIGS. 55 to57 and FIG. 58 the voltage to be applied during the trial jetting of inkhas been described as that employed during the actual recordingoperation, the voltage to be applied during the trial jetting of ink maybe higher than that applied during the actual recording operation sothat cleaning of the nozzles can be facilitated by removing clogged inkfrom the nozzle. For this purpose, there is provided a circuitry shownin FIGS. 59 and 60, which will now be described.

Referring now to FIGS. 59 and 60, the high voltage power source, whichhas been described as constituted by the high voltage pulse generator 50and the bias voltage source 30 is shown to additionally include a testvoltage generator 221 connected in a circuit between the high voltagepulse generator 50 and the nozzles 15a to 15c and adapted to receive thetest signal issued from the test signal generator 210, the mode ofoperation of said test voltage generator 221 being controlled by thetest signal from the test signal generator 210. The high voltage powersource shown in FIG. 60 may be employed in the circuitry shown in FIG.58. Moreover, the circuitry of the high voltage power source shown inFIG. 60 is equally applicable to the circuitry shown in any one of FIGS.50 to 53 and FIGS. 55 to 57.

In the circuit arrangement shown in FIG. 17, it has been found that,since the switching voltage to be applied to the nozzle 15 from thevoltage power source including the bias voltage source 30 and the highvoltage pulse generator 50 has a constant, predetermined value, thespeed of transformation of the ink into the fine droplets tends to bereduced, thereby adversely affecting the finish of the ultimatelydelineated line, during a recording operation in which the recordinghead 12 is moved at a relatively high speed.

This can be overcome by employing the high voltage power sourceconstructed as shown in FIG. 61, reference to which will now be made.

Referring now to FIG. 61, in the circuit between the high voltage pulsegenerator 50 and the nozzle 15, is connected a high voltage amplifier222 is inserted and is connected to the output terminal of the negativefeed-back amplifier 39 so that the high voltage amplifier 222 cangenerate, in response to the output from the amplifier 39, a voltage inproportion to the pressure being applied to the ink surface within theink tank 14. The voltage generated by the high voltage amplifier 222 issubsequently superimposed upon the switching voltage outputted from thehigh voltage pulse generator 50 in response to the trigger signal Sd orSs. FIG. 62 is a diagram showing the waveforms of the output from thenegative feed-back amplifier 39, the voltage applied to the nozzle 15from the high voltage amplifier 222 and the trigger signal Sd or Ss.

As hereinbefore described, so long as no trigger signal is applied tothe high voltage pulse generator 50, the pressure acting on the inksurface within the ink tank 14 is zero (i.e., equal to the atmosphericpressure), the level of the output from the pressure sensor 38 is alsozero and, therefore, the level of the output from the high voltageamplifier 222 is zero. In this condition, only the bias voltage Vol₁from the bias voltage source 30 is applied to the nozzle 15 and,therefore, the meniscus of ink is formed at the tip of the nozzle 15 inreadiness for the jetting thereof towards the recording medium 21.

When the trigger signal Sd or Ss is subsequently applied to the highvoltge pulse generator 50, the latter generates the switching voltageVol₂ which is superimposed upon the bias voltage Vol₁ and is thenapplied to the nozzle 15 to effect the jetting of ink from the nozzle 15towards the recording medium 21. Simultaneously therewith, eachcomponent of the compressed air source 42 is brought into operation andthe output signal from the pressure sensor 38, which is indicative ofthe pressure acting on the ink surface within the ink tank 14, isapplied through the negative feed-back amplifier 39 to the voltageamplifier 222 which serves to superimpose a negative fed-back voltageVol₃ from the amplifier 39 on the switching voltage, which has alreadybeen superimposed upon the bias voltage, in correspondence to thepressure detected by the pressure sensor 38. This fed-back voltage Vol₃varies with the change in pressure of the compressed air being suppliedto the ink tank 14 and, accordingly, varies in proportion to therecording velocity and the amount of ink jetted.

From the foregoing, it is clear that, since the voltage to be applied tothe nozzle 15 is variable in response to the change in pressure of thecompressed air fed to the ink tank 14, transformation of the inkmeniscus into the fine droplets and then into a jet of ink can readilytake place.

In practice, the high voltage power source having the circuitarrangement shown in FIG. 60 is constructed as shown in FIG. 63.

Referring to FIG. 63, the bias voltage source 30 is shown to be composedof a D.C./D.C. converter 230 having an input terminal connected to aD.C. power source E of 24 volts and an output terminal connected to avoltage multiplying and rectifying circuit 231. This bias voltage source30 is so designed that the voltage from the power source E can beamplified by the converter 230 to about 300 volts which is furtheramplified to about 1,100 volts by the multiplying and rectifying circuit231 which is composed of four voltage multiplying and rectifying circuitcomponents connected in cascade.

The high voltage pulse generator 50 is shown to be composed of aD.C./D.C. converter 232, a voltage multiplying and rectifying circuit233, and a high voltage switching circuit 234 all connected in serieswith each other, the input terminal of said converter 232 beingconnected to the common D.C. power source E. The switching circuit 234is adapted to receive the trigger signal Sd or SS by which the mode ofoperation of said switching circuit 234 is controlled to selectivelyswitch the output voltage therefrom off and on.

The high voltage amplifier 222 is shown to be composed of a D.C./D.C.converter 235 for amplifying the voltage from the common power source Eto 300 volts and a voltage multiplying and rectifying circuit 236,composed of a single voltage multiplying and rectifying circuitcomponent, for amplifying the voltage from the converter 235 to about500 volts which is subsequently supplied to a D.C. amplifier 237 whichgenerates an output signal in response to the pressure signal appliedthereto from the pressure sensor 38 through the feed-back amplifier 39.

The voltage multiplying and rectifying circuit 231, the switchingcircuit 234 and the D.C. amplifier 237 are connected in series from theground so that the output voltage emerging from the amplifier 237 andsupplied to the nozzle 15 has a waveform such as shown in FIG. 62. It isclear that the output voltage applied to the nozzle 15 from theamplifier 237 has a waveform corresponding to that of the fed-backpressure signal.

It is to be noted that input stages of the switching circuit 234 and theD.C. amplifier 237 employ respective photo-couplers PC₁ and PC₂ as shownin FIGS. 65 and 66. By the use of the photo-coupler PC₁ and PC₂, anypossible breakage of the circuit 234 or 237 resulting from theapplication of the high voltage thereto can advantageously be avoided.In the circuit shown in FIG. 63, the input stage of the amplifier 237and that of the switching circuit 234 require a break-down voltage ofabout 2.2 Kv. or more and 2.7 Kv. or more, respectively.

If the order of connection of the voltage multiplying and rectifyingcircuit 231, the switching circuit 234 and the amplifier 237, shown inFIG. 63, is reversed such as shown in FIG. 64 in consideration of thebreak-down voltage of each of the photo-couplers PC₁ and PC₂ (andaccordingly, the bias voltage source 30 and the high voltage amplifier222 shown in FIG. 61 have their positions reversed with respect to thehigh voltage pulse generator 50), the individual break-down voltages ofthe input stages of the D.C. amplifier 237 and the voltage multiplyingand rectifying circuit 231 can be reduced to 1.6 Kv. or more and 500 v.or more, respectively, although the output voltage emerging from theamplifier 237 and applied to the nozzle 15 has substantially the samewaveform as shown in FIG. 62.

Although the present invention has fully been described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications will be apparent to those skilled in the art. By way ofexample, the ink jet recording apparatus according to the presentinvention can equally be applicable to any type of X-Y plotter havingdigital-controlled X-axis and Y-axis servomotors.

Accordingly, such changes and modifications are to be understood asbeing included within the true scope of the present invention unlessthey depart thereform.

What is claimed is:
 1. An ink jet recording apparatus having at leastone ink tank for containing therein a sustantial amount of ink, a nozzleconnected to said ink tank for discharging the ink within the ink tankto the outside of said ink tank, an electrode supported in position inspaced relation to said nozzle, means connected to said nozzle and saidelectrode for applying a voltage between said nozzle and said electrodefor establishing an electric field between said nozzle and saidelectrode, said nozzle and voltage applying means having a shape andapplying a sufficient voltage for producing an electric field having anintensity which is symmetrical with respect to the center of the nozzlefor causing the ink droplets to travel straight from the nozzle andhaving a strength for causing the ink to be drawn out of the nozzle dueto the field alone, means connected to said nozzle for moving the nozzlerelative to a recording medium with the tip of the nozzle spaced apredetermined distance from the recording medium, and means forcontrolling the amount of ink jetted from the nozzle towards therecording medium, said controlling means comprising;pressure controlmeans connected to said tank for controlling the static air pressureacting on the surface of the ink within the ink tank to an amount lessthan that necessary to jet ink from the nozzle solely by the airpressure and for varying the static air pressure for varying the amountof ink jetted from the nozzle in proportion to the air pressure;pressure sensor means connected to said tank for detecting the airpressure acting on the surface of the ink within the ink tank and forgenerating an output signal indicative of the value of the pressure thusdetected; signal generating means for generating a signal correspondingto the value of an air pressure equivalent to an amount of the ink whichis desired to be jetted from the nozzle in relation to the operation ofthe ink jet recording apparatus in order to draw a line of apredetermined width; comparison means connected to said signalgenerating means and said pressure sensor means for comparing the outputsignal from said pressure sensor means with the signal from said signalgenerating means for generating an output signal indicative of anydifference therebetween; and operating means connected between saidcomparison means and said pressure control means for operating thepressure control means in response to the output signal from saidcomparison means, whereby the ink jetted from the nozzle is controlledto draw a line of predetermined width.
 2. An ink jet recording apparatushaving at least one ink tank for containing therein a substantial amountof ink, a nozzle connected to said ink tank for discharging the inkwithin the ink tank to the outside of said ink tank, an electrodesupported in position in spaced relation to said nozzle, means connectedto said nozzle and said electrode for applying a voltage between saidnozzle and said electrode for establishing an electric field betweensaid nozzle and said electrode, said nozzle and voltage applying meanshaving a shape and applying a sufficient voltage for producing anelectric field having an intensity which is symmetrical with respect tothe center of the nozzle for causing the ink droplets to travel straightfrom the nozzle and having a strength for causing the ink to be drawnout of the nozzle due to the field alone, means connected to said nozzlefor moving the nozzle relative to a recording medium with the tip of thenozzle spaced a predetermined distance from the recording medium,velocity detecting means associated with said nozzle and said recordingmedium for detecting the velocity of said relative movement of thenozzle and the recording medium for generating an output signalindicative of the detected velocity, and means for controlling theamount of ink jetted from the nozzle towards the recording medium fordrawing lines of predetermined width, said controlling meanscomprising;pressure control means connected to said tank for controllingthe static air pressure acting on the surface of the ink within the inktank to an amount less than that necessary to jet ink from the nozzlesolely by the air pressure and for varying the static air pressure andfor varying the amount of ink jetted from the nozzle in proportion tothe air pressure; pressure sensor means connected to said tank fordetecting the air pressure acting on the surface of the ink within theink tank and for generating an output signal indicative of the value ofthe pressure thus detected; comparison means connected to said pressuresensor means and said velocity detecting means for comparing the outputsignal from the pressure sensor means with the output signal from thevelocity detecting means and for generating an output signal indicativeof any difference therebetween; and operating means connected betweensaid comparison means and said pressure control means for operating thepressure control means in response to the output signal from saidcomparison means, whereby the ink jetted from the nozzle is controlledin accordance with the variance of the relative velocity of the nozzleand the recording medium.
 3. An apparatus as claimed in claim 2, whereinthe pressure control means comprises a pulse motor, the angle ofrotation of which is controlled in accordance with the output signalfrom the comparison means, an air pump having a piston memberoperatively coupled to said pulse motor, and a flexible tubing forsupplying compressed air from the air pump to the ink tank.
 4. Anapparatus as claimed in claim 2, further comprising a detector fordetecting a reduction of the amount of the ink within the ink tank andfor generating an output control signal and connected to said pressurecontrol means for controlling the pressure of the compressed air forcompensating for a reduction in the static pressure of the ink withinthe ink tank resulting from such reduction of the amount of the inkwithin the ink tank.
 5. An apparatus as claimed in claim 2, furthercomprising a temperature sensor in said ink tank for detecting thetemperature of the ink within the ink tank and for generating a controlsignal and connected to said pressure control means for controlling thepressure control means for avoiding variation in the amount of the inkjetted which may result from change in viscosity of the ink due tochange in temperature of the ink within the ink tank.
 6. An apparatus asclaimed in claim 2, further comprising a stabilizer accommodated withinthe ink tank for suppressing motion of the body of the ink within theink tank which may otherwise result during the relative movement of thenozzle and the recording medium.
 7. An apparatus as claimed in claim 2,further comprising a line-width selection signal generating means, andmeans connected to said line width selection signal generating means andconnected between said pressure sensor means and said comparison meansfor varying the gain of the output signal from the pressure sensor meansand supplied to the comparison means.
 8. An apparatus as claimed inclaim 7, wherein said varying means includes an amplifier, the gain ofwhich is adjustable and to which the output signal from the pressuresensor means is applied, and said line-width selection signal generatingmeans includes means for controlling the gain of the amplifier forvarying the output signal from the pressure sensor means in accordancewith the gain to cause the output signal from the comparing means tovary for thereby operating the pressure control means so as to adjustthe pressure of the compressed air supplied to the ink tank so that aline of desired width can be delineated on the recording medium by thejet of ink issued from the nozzle.
 9. An apparatus as claimed in claim 2further comprising a line-width selection signal generating means andmeans connected to said line-width selection signal generating means andconnected between said velocity detecting means and said comparisonmeans for varying the gain of the control signal supplied to saidcomparison means from said detecting means.
 10. An apparatus as claimedin claim 9, wherein said varying means includes an amplifier, the gainof which is adjustable and to which the output signal from the velocitydetecting means is applied, and said line-width selection signalgenerating means includes a means for controlling the gain of theamplifier for varying the velocity signal in accordance with the gain tocause the output signal from the comparing means to vary for therebyoperating the pressure control means so as to adjust the pressure of thecompressed air supplied to the ink tank so that a line of desired widthcan be delineated on the recording medium by the jet of ink issued fromthe nozzle.
 11. An apparatus as claimed in claim 2, wherein said voltageapplying means comprises a bias voltage source for generating a biasvoltage for forming a meniscus of ink at the tip of the nozzle and aswitching voltage source for generating a switching voltage necessary todraw the ink droplets successively from the tip of the nozzle.
 12. Anapparatus as claimed in claim 11, wherein the voltage applying meansfurther includes means connected to said pressure sensor means forfurther applying between the nozzle and the principal electrode avoltage proportional to the output signal from the pressure sensor meansduring the application of the switching voltage.
 13. An ink jetrecording apparatus having at least one ink tank for containing thereina substantial amount of ink, a nozzle connected to said ink tank fordischarging the ink within the ink tank to the outside of the ink tank,an electrode supported in position in spaced relation to said nozzle,means connected to said nozzle and said electrode for applying a voltagebetween the nozzle and the electrode for establishing an electric fieldbetween the nozzle and the electrode, said nozzle and voltage applyingmeans having a shape and applying a sufficient voltage for producing anelectric field having an intensity which is symmetrical with respect tothe center of the nozzle for causing the ink droplets to travel straightfrom the nozzle and having a strength for causing the ink to be drawnout of the nozzle due to the field alone, means connected to the nozzlefor moving the nozzle relative to a recording medium, and means forcontrolling the amount of ink jetted from the nozzle towards therecording medium, said controlling means comprising;pressure controlmeans connected to said tank for controlling the static air pressureacting on the surface of the ink within the ink tank to an amount lessthan that necessary to jet ink from the nozzle solely by the airpressure and for varying the static air pressure and for varying theamount of ink jetted from the nozzle in proportion to the air pressure;pressure sensor means connected to said tank for detecting the pressureacting on the surface of the ink within the ink tank and for generatingan output signal indicative of the pressure thus detected; meansassociated with said nozzle and the recording medium for detecting thevelocity of relative movement of the nozzle and the recording medium andfor generating an output signal proportional to the detected velocity;comparison means connected to said pressure sensor means and saidvelocity detecting means for comparing the output signal from thevelocity detecting means for generating an output signal indicative ofany difference therebetween; and means connected between said comparisonmeans and said pressure control means for operating the pressure controlmeans in response to the output signal from said comparing means; saidpressure control means having a pulse motor, the angle of rotation ofwhich is controlled by the output signal from the operating means,motion translator means connected to the pulse motor for translating therotation of the pulse motor into a linear motion, an air pump includinga sealed cylinder and a piston operatively accommodated within saidcylinder for adjusting the volume of the interior of said cylinder,means connected between the motion translator means and the piston fortransmitting the linear motion to the piston, and a flexible tubinghaving its ends connected to the ink tank and the cylinder,respectively, for transmitting the pressure evolved within the cylinderby said piston to the ink tank whereby the ink jetted from the nozzle iscontrolled in accordance with the pressure transmitted to the ink tank.14. An apparatus as claimed in claim 13, wherein the motion translatingmeans comprises an eccentric cam member rotatable together with therotation of the pulse motor and a rod member coupled to said eccentriccam for reciprocal movement in a linear direction in synchronism withthe rotation of the eccentric cam.